diff --git a/README.md b/README.md
index e45a4ec..33f7750 100644
--- a/README.md
+++ b/README.md
@@ -373,7 +373,9 @@ game:
   the game is out of print and the rulebook could not be obtained, so the
   document says which parts are attested and which were derived
 - [Katan](docs/RULES-catan.md) — the island game (catalogue id `catan`); the
-  rules engine and AI are in place, the screen lands with its own task
+  rules engine, AI and screen are in place
+- [Katan: Cities & Knights](docs/RULES-catan-ck.md) — the expansion rule set, chosen on
+  the setup screen: commodities, knights, barbarians, city improvements and progress cards
 
 ## Layout
 
@@ -396,7 +398,7 @@ game/          rules engines and AI — pure Kotlin, no Android, unit-tested
   mastermind/  Mastermind codes, scoring, and a code breaker
   pirates/     Pirates and Bulgars — the cross board, the hunt, search
   tayu/        Ta Yü tile geometry, river rules, scoring, AI
-  catan/       Katan island geometry, production, trade, dev cards, heuristic AI
+  catan/       Katan island geometry, production, trade, dev cards, Cities & Knights, heuristic AI
 net/           wire protocol, LAN transport, lobby, match driver
 score/         scorekeeper sheet — pure Kotlin, unit-tested — and its store
 settings/      DataStore-backed preferences
diff --git a/app/src/main/java/org/prolibertate/games/game/Catalog.kt b/app/src/main/java/org/prolibertate/games/game/Catalog.kt
index abb732a..fa63168 100644
--- a/app/src/main/java/org/prolibertate/games/game/Catalog.kt
+++ b/app/src/main/java/org/prolibertate/games/game/Catalog.kt
@@ -360,7 +360,7 @@ object GameCatalog {
             minPlayers = 3,
             maxPlayers = 4,
             teamBased = false,
-            blurb = "Trade, build, and settle the island.",
+            blurb = "Trade, build, and settle the island, in the base game or Cities & Knights.",
             // Engine and screen exist but have not been verified on two real
             // devices yet. Dev builds can open it as a *Preview*; a production
             // build does not list it. Flip `available` on, and `devPreview`
diff --git a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanInteraction.kt b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanInteraction.kt
index f347f92..c62f975 100644
--- a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanInteraction.kt
+++ b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanInteraction.kt
@@ -71,11 +71,14 @@ data class BoardTargets(
     val edges: Set<Int> = emptySet(),
     val hexes: Set<Int> = emptySet(),
     val knights: Set<Int> = emptySet(),
+    /** The player's other knights: tappable for their menu, but with nothing to do, so not ringed. */
+    val idleKnights: Set<Int> = emptySet(),
 ) {
-    val isEmpty: Boolean get() = vertices.isEmpty() && edges.isEmpty() && hexes.isEmpty() && knights.isEmpty()
+    val isEmpty: Boolean
+        get() = vertices.isEmpty() && edges.isEmpty() && hexes.isEmpty() && knights.isEmpty() && idleKnights.isEmpty()
 
     /** Every crossing a tap can land on. */
-    val tappableVertices: Set<Int> get() = vertices + knights
+    val tappableVertices: Set<Int> get() = vertices + knights + idleKnights
 }
 
 /**
@@ -143,6 +146,8 @@ object CatanInteraction {
         setupVertex: Int?,
         movingKnight: Int? = null,
         armed: ArmedCard? = null,
+        /** The local seat, so its sleeping knights can be tapped for their menu too. */
+        localSeat: Int? = null,
     ): BoardTargets {
         val setup = legal.filterIsInstance<SetupPlace>()
         if (setup.isNotEmpty()) {
@@ -187,7 +192,15 @@ object CatanInteraction {
             BuildKind.KNIGHT ->
                 BoardTargets(vertices = legal.filterIsInstance<HireKnight>().map { it.vertex }.toSet())
 
-            null -> BoardTargets(knights = knightsWithActions(legal))
+            null -> {
+                val ready = knightsWithActions(legal)
+                val mine = if (localSeat != null && legal.isNotEmpty()) {
+                    state.ck?.knights?.filter { it.owner == localSeat }?.map { it.vertex }?.toSet().orEmpty()
+                } else {
+                    emptySet()
+                }
+                BoardTargets(knights = ready, idleKnights = mine - ready)
+            }
         }
     }
 
@@ -323,6 +336,8 @@ object CatanInteraction {
         setupVertex: Int?,
         movingKnight: Int? = null,
         armed: ArmedCard? = null,
+        /** The local player's knights, which can be tapped for their menu even with nothing to do. */
+        ownKnights: Set<Int> = emptySet(),
     ): TapOutcome {
         val setup = legal.filterIsInstance<SetupPlace>()
         if (setup.isNotEmpty()) {
@@ -350,7 +365,7 @@ object CatanInteraction {
             BuildKind.CITY -> BuildCity(vertex)
             BuildKind.WALL -> BuildWall(vertex)
             BuildKind.KNIGHT -> HireKnight(vertex)
-            else -> return if (building == null && vertex in knightsWithActions(legal)) {
+            else -> return if (building == null && (vertex in knightsWithActions(legal) || vertex in ownKnights)) {
                 TapOutcome.PickKnight(vertex)
             } else {
                 TapOutcome.Ignored
diff --git a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanPanels.kt b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanPanels.kt
index 59aaa10..e23a986 100644
--- a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanPanels.kt
+++ b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanPanels.kt
@@ -239,7 +239,7 @@ fun CatanPlayerCard(
             } else {
                 CardLine("$cards cards · ${ck.progressCounts[seat]} progress", MaterialTheme.typography.bodySmall)
                 CardLine(
-                    "Knights ${CatanCk.knightsOf(state, seat).size} (strength $strength) · " +
+                    "⚔ ${CatanCk.knightsOf(state, seat).size} · str $strength · " +
                         "Road ${CatanRules.longestRoadFor(state, seat)}",
                     MaterialTheme.typography.bodySmall,
                 )
diff --git a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanScreen.kt b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanScreen.kt
index cff6723..32b98d9 100644
--- a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanScreen.kt
+++ b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanScreen.kt
@@ -117,7 +117,6 @@ fun CatanScreen(
             setupVertex = null
         }
         if (movingKnight != null && legal.none { it is MoveKnight && it.from == movingKnight }) movingKnight = null
-        if (knightMenu != null && CatanInteraction.knightActions(legal, knightMenu!!).isEmpty()) knightMenu = null
         armed?.let { card -> if (card.plays.none { it in legal }) armed = null }
         if (legal.isEmpty()) {
             showCards = false
@@ -159,7 +158,8 @@ fun CatanScreen(
         return
     }
 
-    val targets = CatanInteraction.targets(current, legal, building, setupVertex, movingKnight, armed)
+    val targets = CatanInteraction.targets(current, legal, building, setupVertex, movingKnight, armed, localSeat)
+    val ownKnights = current.ck?.knights?.filter { it.owner == localSeat }?.map { it.vertex }?.toSet().orEmpty()
     val devHand = CatanInteraction.devHand(current, localSeat, legal)
     val progressHand = CatanInteraction.progressHand(current, localSeat, legal)
     val playing = current.phase == CatanPhase.PLAYING && legal.isNotEmpty()
@@ -219,7 +219,12 @@ fun CatanScreen(
                         targets = targets,
                         selectedVertex = setupVertex,
                         onVertexTap = {
-                            handle(CatanInteraction.onVertexTap(it, legal, building, setupVertex, movingKnight, armed))
+                            handle(
+                                CatanInteraction.onVertexTap(
+                                    it, legal, building, setupVertex, movingKnight, armed,
+                                    ownKnights = if (legal.isNotEmpty()) ownKnights else emptySet(),
+                                ),
+                            )
                         },
                         onEdgeTap = { handle(CatanInteraction.onEdgeTap(it, legal, building, setupVertex, armed)) },
                         onHexTap = { handle(CatanInteraction.onHexTap(it, legal, armed)) },
diff --git a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanStyle.kt b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanStyle.kt
index 90db565..7cc53c6 100644
--- a/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanStyle.kt
+++ b/app/src/main/java/org/prolibertate/games/ui/game/catan/CatanStyle.kt
@@ -58,8 +58,8 @@ data class CatanStyle(
     /** Wall round a city, and the barbarian strip. */
     val wall: Color = Color(0xFF5D4037),
     val barbarian: Color = Color(0xFFB71C1C),
-    /** A knight that is awake and ready to act. */
-    val knightReady: Color = Color(0xFFFFEB3B),
+    /** The halo round a knight that is awake. White, so it is not mistaken for the yellow tap ring. */
+    val knightReady: Color = Color(0xFFFFFFFF),
 
     // --- Players ------------------------------------------------------------
     /** One colour per seat, in seat order. Roads, settlements, cities, player strip. */
diff --git a/app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkInteractionTest.kt b/app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkInteractionTest.kt
index 4b9fd6f..bc0807c 100644
--- a/app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkInteractionTest.kt
+++ b/app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkInteractionTest.kt
@@ -2,6 +2,7 @@ package org.prolibertate.games.ui.game.catan
 
 import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
 import org.junit.Assert.assertTrue
 import org.junit.Test
 import org.prolibertate.games.game.GameCatalog
@@ -261,4 +262,26 @@ class CatanCkInteractionTest {
         listOf("setup", "road", "build SETTLEMENT", "build CITY", "knightMenu", "improve", "bankTrade", "roll", "endTurn")
             .forEach { assertTrue("never exercised: $it in $total", (total[it] ?: 0) > 0) }
     }
+
+    @Test
+    fun `a sleeping knight with nothing to do can still be tapped for its menu`() {
+        val base = CatanRules.initialState(config(3, 5L))
+            .copy(phase = CatanPhase.PLAYING, turn = 0, turnNumber = 1)
+        val state = base.copy(
+            ck = base.ck!!.copy(knights = listOf(org.prolibertate.games.game.catan.Knight(10, 0, 1))),
+        )
+        val legal = CatanRules.legalMoves(state, 0)
+        assertTrue("no grain, so nothing to do", CatanInteraction.knightActions(legal, 10).isEmpty())
+
+        val targets = CatanInteraction.targets(state, legal, null, null, localSeat = 0)
+        assertTrue(10 in targets.idleKnights)
+        assertTrue("no tap ring for a knight with nothing to do", 10 !in targets.knights)
+        assertTrue(10 in targets.tappableVertices)
+        assertEquals(
+            TapOutcome.PickKnight(10),
+            CatanInteraction.onVertexTap(10, legal, null, null, ownKnights = setOf(10)),
+        )
+        // On somebody else's turn the legal list is empty and nothing is offered.
+        assertTrue(CatanInteraction.targets(state, emptyList(), null, null, localSeat = 0).idleKnights.isEmpty())
+    }
 }
diff --git a/docs/README.md b/docs/README.md
index d6ef5a9..3a93b26 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -24,7 +24,7 @@ Naming: the catalogue id, lowercase, e.g. `crazy8s` → `RULES-crazy8s.md`.
 | Golf | [RULES-golf.md](RULES-golf.md) |
 | Hearts | — (TODO) |
 | Kaiser | [RULES-kaiser.md](RULES-kaiser.md) |
-| Katan (catalogue id `catan`) | [RULES-catan.md](RULES-catan.md) |
+| Katan (catalogue id `catan`) | [RULES-catan.md](RULES-catan.md) — and its expansion rule set, [RULES-catan-ck.md](RULES-catan-ck.md) |
 | Klondike | — (TODO) |
 | Mastermind | [RULES-mastermind.md](RULES-mastermind.md) |
 | Morris | [RULES-morris.md](RULES-morris.md) |
diff --git a/docs/RULES-catan-ck.md b/docs/RULES-catan-ck.md
new file mode 100644
index 0000000..70029bb
--- /dev/null
+++ b/docs/RULES-catan-ck.md
@@ -0,0 +1,220 @@
+# Katan: Cities & Knights — implemented rules
+
+This is the second rule set of Katan (see [RULES-catan.md](RULES-catan.md) for the
+base game). It is chosen on the setup screen — **Rules: Base game / Cities &
+Knights** — and stored as `CatanOptions.ruleSet` (`CatanRuleSet.CITIES_AND_KNIGHTS`).
+A table plays one or the other; the choice reaches guests with the rest of the
+options.
+
+The expansion is a whole rule set rather than a toggle, because it replaces rules
+(no development cards, no Largest Army, a different seven) as well as adding them.
+Everything it needs lives behind `CatanState.ck`, which is `null` in the base game,
+so a base game's state and rules are untouched. The code is:
+
+| File | Holds |
+| --- | --- |
+| `CatanCkModel.kt` | the data: commodities, improvement tracks, progress cards, knights, the chore queue, every new move |
+| `CatanCk.kt` | the rules: legal moves, applying them, the roll, the barbarians, scoring |
+| `CatanCkAi.kt` | the computer opponent for this rule set |
+| `CatanRules.kt` | the single entry point; calls into `CatanCk` whenever `state.ck != null` |
+
+Behaviour is pinned by `CatanCkRulesTest` (each rule on a crafted position) and
+`CatanCkSelfPlayTest` (whole games, engine against engine). Three or four players,
+as in the base game.
+
+## Verification
+
+The rules were checked against the in-game almanac of Catan Universe (rules text
+only: costs, card wordings, the barbarian and improvement rules). Where the almanac
+is silent or ambiguous the reading chosen is listed under *Choices made here* below.
+
+## What changes from the base game
+
+| | Base game | Cities & Knights |
+| --- | --- | --- |
+| Points to win | 10 | **13** (setup offers 8 / 10 / 13 / 15) |
+| Development cards | 25-card deck | **none** — knights and progress cards replace them |
+| Largest Army | 5 *(see RULES-catan.md)* | **none** |
+| Longest Road | 5 *(see RULES-catan.md)* | **2** |
+| Second setup piece | settlement | **city** (resources only, no commodities) |
+| Dice | two | **two number dice + the event die** |
+| A seven | discard half over 7; robber moves | discard half over **7 + 2 per wall**; the robber **only moves after the first barbarian attack** |
+| Cities | 2 of a resource | forest: 1 lumber + **1 paper**; pasture: 1 wool + **1 cloth**; mountains: 1 ore + **1 coin**; hills / fields: 2 of the resource |
+
+## The dice
+
+The yellow die and the red die are added for production, as in the base game. The
+**event die** has three ships and one gate each of yellow (trade), blue (politics)
+and green (science). It is resolved **before** production:
+
+- **Ship** — the barbarians come one step nearer (they start 7 steps out).
+- **Gate** — every player with at least one level of improvement in that colour,
+  **in turn order from the roller**, whose **red die is no higher than their level**
+  in that track, draws a progress card of that colour.
+
+`RollDice(forced, event)` stages both for tests, like the base game's forced roll.
+An **Alchemist** played before the roll sets the two number dice (the event die is
+still rolled).
+
+## Commodities
+
+Three commodities — **cloth, coin, paper** — join the five resources. The bank holds
+12 of each (19 of each resource). Hands, discards, thefts and trades all treat them as
+cards; progress cards are not cards for any of these purposes.
+
+When the bank cannot meet the demand for a card type: with **one** seat owed it,
+that seat takes what is left; with **two or more** owed, nobody receives it.
+
+## The barbarians
+
+The strip shows how many steps away the ship is. On the seventh ship it **lands**:
+
+- Barbarian strength is the **number of cities on the board** (a metropolis is a city).
+- Catan's strength is the sum of the strengths of every **active** knight (basic 1,
+  strong 2, mighty 3).
+- If Catan's strength is **at least** the barbarians', Catan holds: the player with
+  the strictly highest contribution takes a **Defender of Catan** card (one point);
+  if several tie for highest, **each draws a progress card from a deck of their
+  choice** instead.
+- Otherwise Catan falls: among the players who own a city that is not a metropolis,
+  those who contributed **least** each lose one city, **to their own choice**. A lost
+  city becomes a settlement (or, if the seat has no settlement piece left, is removed),
+  and its wall goes. **A metropolis can never be pillaged.**
+- Either way every knight stands down, the ship returns to seven steps, and the
+  **robber wakes** — until now it has not been on the board.
+
+## Knights
+
+Each seat has **two knights of each strength** (six in all).
+
+| Action | Cost | Rule |
+| --- | --- | --- |
+| Hire | 1 ore + 1 wool | a **basic** knight on an empty crossing at the end of one of your roads; only if one of your two basic knights is free |
+| Activate | 1 grain | a sleeping knight wakes; only active knights count against the barbarians |
+| Promote | 1 ore + 1 wool | one level, once per knight per turn; a **mighty** knight needs **level three of politics** (the fortress); the next level's pieces must be free |
+| Move | — | an active knight moves along **its own roads** to an empty crossing, or onto a **strictly weaker** foreign knight, driving it off; it then rests |
+| Chase the robber | — | an active knight on a crossing of the robber's hex sends the robber off (only once the robber is awake); it then rests, and the robber move follows |
+
+- **A knight cannot act on the turn it woke.** It can be woken again after it acts.
+- A driven-off knight is moved by **its owner** along its own roads to any empty
+  crossing it can reach; if there is none, it is lost.
+- A knight is a wall, like an opposing building: roads cannot be laid past an enemy
+  knight, a settlement cannot be built on a knight, and the knight cuts **Longest Road**.
+- Knights can act (wake, move, chase) **before the roll** as well as after.
+
+## City walls
+
+2 brick, on a city without a wall, **three per player**. Each wall raises the hand
+limit on a seven by two (so a seven discards when you hold more than `7 + 2 × walls`).
+
+## City improvements
+
+Three tracks, five levels each, bought with the track's commodity: **trade (cloth,
+yellow), politics (coin, blue), science (paper, green)**. Level *n* costs *n* of the
+commodity. You must own a city to improve.
+
+| Track | Level 3 gives |
+| --- | --- |
+| Trade | commodities trade with the bank **2:1** |
+| Politics | knights may be promoted to **mighty** |
+| Science | the **aqueduct**: when the dice pay you nothing, take any resource |
+
+The level names (Trading Post … Grand Bazaar, Town Hall … Palace, Abbey … University)
+are this app's own.
+
+### Metropolises
+
+The first player to reach **level four** in a track takes that track's **metropolis**
+(a marker on one of their cities: +2 points; a city carries at most one). Reaching
+**level five** takes it from a holder who is still on four. A player with several
+cities chooses which; with one it is placed at once; a player with no free city
+**waits**, and claims it when a city is built.
+
+## Progress cards
+
+54 cards: 18 in each of three decks. A card played returns to the **bottom** of its
+deck. A hand holds **four**; a fifth must be given back at once. They can be played at
+any point of your own turn (before or after the roll), except where noted.
+
+**Science (green)** — Alchemist ×2 *(before the roll only)*, Crane ×2, Engineer, Inventor ×2,
+Irrigation ×2, Medicine ×2, Mining ×2, Printer, Road Building ×2, Smith ×2.
+
+**Politics (blue)** — Bishop ×2 *(after the first attack)*, Constitution, Deserter ×2,
+Diplomat ×2, Intrigue ×2, Saboteur ×2, Spy ×3, Warlord ×2, Wedding ×2.
+
+**Trade (yellow)** — Commercial Harbor ×2, Master Merchant ×2, Merchant ×6,
+Merchant Fleet ×2, Resource Monopoly ×4, Trade Monopoly ×2.
+
+**Printer** and **Constitution** are one point each and are **laid face up the moment
+they are drawn**; they never sit in a hand.
+
+How each plays is shown on the card in the app (`CatanText.progressHelp`); the engine's
+behaviour is in `CatanCk.applyProgress`. Points worth knowing:
+
+- **Saboteur** hits every other player with **as many points as you or more**, who each
+  discard half their cards (rounded down). **Wedding** takes two cards (or what they
+  have) from each player with **more** points. **Master Merchant** needs a victim with
+  more points, and takes two cards you choose.
+- **Spy**: you see the target's progress cards and take one; nobody else sees them.
+- **Merchant**: placed beside one of your buildings; you trade that hex's resource 2:1
+  and hold a victory point until someone else plays a Merchant.
+- **Deserter**: the victim removes a knight of *their* choice; you place a free one of the
+  same strength (or a basic if yours is not free), awake or asleep as theirs was.
+- **Diplomat**: removes a road with a loose end (an end touching no other road, building
+  or knight); if it was yours you may rebuild it anywhere for free.
+- **Inventor** swaps two tokens, never a 2, 6, 8 or 12.
+
+## The chore queue
+
+Anything that needs an answer from somebody — a discard, a city to lose, a progress card
+to hand over, a metropolis site, a knight to place — is queued as a **chore**
+(`CatanPhase.CHORE`, `CkState.chores`). The game waits on the seat the first chore names
+(not always the player whose turn it is), then resumes where `CkState.back` says. This is
+how a seven, a barbarian landing, a wedding or a spy are all one mechanism, and why
+the AI and the network driver needed no new concepts.
+
+The two free-form chores — **discarding** and **handing over** cards — are structural
+like the base game's trade offers: the engine checks them in `isLegalMove` and the legal
+list carries one example, so a seat is never without a move.
+
+## Scoring and winning
+
+Settlement 1, city 2, **metropolis +2**, **Longest Road 2**, **Defender of Catan 1** each,
+**Printer / Constitution 1** each, **the Merchant 1**. Nothing is hidden. Win at 13 (default),
+checked after every move and when a turn arrives.
+
+## Choices made here, and why
+
+- **The red die must be no higher than the improvement level** to draw a progress card
+  (level 1 draws on a 1, level 5 on 1–5). The almanac says the red die must match a
+  number on one of your improvements and that the number rises with the level; this is
+  the reading that makes both true.
+- **One resource is offered to every player for a Commercial Harbor.** The printed card
+  lets you offer a different one to each.
+- **The merchant cannot go on the desert**, which produces nothing to trade.
+- **Improvements need a city** to be built.
+- **A 3:1 harbour trades commodities too**; a 2:1 harbour trades only its own resource.
+- **A knight that cannot find a refuge is lost**, as the almanac says of an intrigue.
+- **A claim on a metropolis with no city to carry it waits** and is made when a city is
+  built.
+- **Domestic trade** may include commodities; the one-offer-at-a-time rules of the base
+  game apply.
+- **No Seafarers, no 5–6 player extension**, no scenario or variant of any kind.
+
+## The computer opponent
+
+`CatanCkAi` plays a safe, unambitious game: it settles and raises cities as the base AI
+does, spends commodities on improvements as soon as it can (leaning to politics), hires
+and wakes knights as the barbarians draw near, chases the robber off hexes it needs, plays
+the progress cards whose value is obvious, and answers every chore with the card it values
+least. It never plans a knight's route, never plays an Alchemist, Inventor, Diplomat,
+Deserter or Intrigue, and makes no domestic offers (though it answers them). Every engine
+game in `CatanCkSelfPlayTest` ends in a win at thirteen points.
+
+## Known gaps
+
+- The screen is built and its logic is tested, but **not yet verified on two real
+  devices**; the catalogue entry stays a dev-channel preview.
+- No animation for the event die, the barbarian landing or a knight's move.
+- The Commercial Harbor offers a single resource type (see above).
+- The AI is not a strong opponent and makes no claim to be.
diff --git a/docs/RULES-catan.md b/docs/RULES-catan.md
index c90c0e5..b4c07a7 100644
--- a/docs/RULES-catan.md
+++ b/docs/RULES-catan.md
@@ -12,14 +12,27 @@ players, as published.
 The engine is a pure state machine (`CatanRules`) with no Android types, and its
 behaviour is pinned by `CatanBoardTest`, `CatanRulesTest` and `CatanAiTest`.
 
+## Rule sets
+
+A table plays one of two rule sets, chosen on the setup screen
+(`CatanOptions.ruleSet`):
+
+- **Base game** — everything in this document.
+- **Cities & Knights** — the expansion, documented in
+  [RULES-catan-ck.md](RULES-catan-ck.md). It reuses this document's board, setup,
+  building, robber and trade rules and changes or replaces the rest; its state lives
+  behind `CatanState.ck`, which is `null` in the base game.
+
 ## Table
 
+- **Rules — *toggle*.** Base game or Cities & Knights (`CatanOptions.ruleSet`).
 - **Players — *toggle*.** Three or four. `CatanOptions.playerCount`; the engine
   refuses anything else.
 - **Points to win — *toggle*.** Defaults to 10. `CatanOptions.pointsToWin`, at
   least 2. A lower target makes a test game short.
 
-The setup screen exposes both (players: 3 or 4; game to 5, 8, 10 or 12 points).
+The setup screen exposes all three (players: 3 or 4; game to 5, 8, 10 or 12 points in the
+base game, 8, 10, 13 or 15 in Cities & Knights, whose published target is 13).
 
 ## Board
 
@@ -296,6 +309,13 @@ timing rules; Longest Road and Largest Army worth five; victory at ten.
 
 ## Known gaps
 
+- **Longest Road and Largest Army are worth 5 each here.** The published base game awards
+  **2** each (Catan Universe's almanac says so for Largest Army, and the printed rules
+  for both). Cities & Knights scores Longest Road at 2, as printed. Left unchanged in
+  the base game so far; it is a one-constant change in `publicVictoryPoints`.
+- **A bank short of a resource pays nobody that roll**, even when only one player is owed
+  it. The published ruling lets a lone claimant take what is left; Cities & Knights does.
+
 - **No domestic auction or counter-offer.** An offer is addressed to a single
   seat. Open trading to the whole table, and counter-offers, are the obvious next
   step and are not implemented.
diff --git a/docs/UI-catan.md b/docs/UI-catan.md
index 6582b20..99be891 100644
--- a/docs/UI-catan.md
+++ b/docs/UI-catan.md
@@ -120,6 +120,34 @@ and play a game on a dev build.
 - The board has a content description but no per-hex semantics; a screen-reader
   mode would need one.
 
+## Cities & Knights on the screen
+
+The same layers carry the expansion; the screen only adds pieces, and **none of them
+decides what is legal** — each reads the engine's legal-move list.
+
+| Layer | What it adds |
+| --- | --- |
+| `CatanStyle.kt` | commodity colours, a colour per improvement track, the wall and barbarian colours, the awake-knight halo |
+| `CatanText.kt` | goods, track, knight and progress-card names and help text; the prompts for every chore |
+| `CatanInteraction.kt` | `BuildKind.WALL` / `KNIGHT`; chore taps; the moving-knight and armed-card modes; `knightActions`, `progressHand`, `goodsBankTrades` |
+| `CatanBoardView.kt` | shields for knights (strength dots; faded when asleep, white halo when awake), dashed walls, a pennant for each metropolis, the merchant coin, a faint robber until the first attack |
+| `CatanPanels.kt` | commodity tiles in the hand, the barbarian strip, the yellow / red / event dice, the extra player-card lines, the Improve and Progress buttons |
+| `CatanCkDialogs.kt` | goods picker (discard, wedding, harbour), improvements, progress cards, the knight menu, deck / aqueduct / spy / drop choices, goods trade |
+| `CatanScreen.kt` | the visual state: knight menu, moving knight, armed card, and which chore dialog shows |
+
+**Interaction model.** Tap one of your own knights (it gets a yellow ring when it can do
+something) for its menu: wake, promote, move or chase the robber. *Move* arms the board
+with the crossings the knight can reach. A progress card that needs a place on the board
+(Engineer, Medicine, Merchant, Diplomat, Intrigue, Inventor) closes the dialog and
+**arms** the board; the next tap plays it. Chores that are answered with a place — a city
+to lose, a metropolis site, a driven knight's refuge, a free knight — light the crossings
+on the board and the prompt line says what to tap; the others open a dialog that cannot
+be dismissed, because the game is waiting on it.
+
+**Testing.** `CatanCkInteractionTest` plays whole games and checks that for *every* legal
+move at *every* position some control (a tap, a button or a dialog choice) produces it, so
+a legal move with no way to make it — a stuck phone — fails the build.
+
 ## Status and known gaps
 
 The screen is **playable and feature-complete for the engine's rules**, but is