Dwarf caves in all modes: neutral dwarves guard a hoard and sometimes a rune chest; the rune forge tower; imps repair towers before claiming

- 1-4 cave slots by map size, 50% each (at least one); knight treasure caves only where a mission places them
- dwarves fight intruders near the cave, never chase far, heal at home, grow with distance and the player's army, never count for victory
- rune chest: one free rune forge tower (chain bolt to three more enemies), upgradeable for gold, sold once it comes back (next mission / free play at once), unbuilt ones carry over
- new tool (key N) shown in the tower menu while a rune tower can be built or upgraded

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

DESIGN.md

@@ -55,7 +55,10 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Chosen in the **New Game dialog**: width and height separately, **64–256 tiles** in steps of 32 (remembered in the settings)
 - Content scales with the area: rock, gold veins and caves; the Hero Gate lies on the far side, the **grace period grows with the distance** to the gate
 - **One portal per 64×64 tiles**; every claimed portal brings creatures (more portals = faster arrivals, still limited by the lair)
-- Bigger maps get **guarded treasure caves** (one less than the number of portals): very rich gold veins, defended by a hero squad that stays at its post
+- **Dwarf caves** in every mode: 1 to 4 slots depending on the map size, each filled with a 50% chance (at least one cave). A hoard (1500-4000 in gold and gems, more the further from the core) inside a cave ringed by rich veins, guarded by 4-6 neutral dwarves (bronze rings). Dwarves fight whoever comes within about 7 tiles of the cave, never chase further, heal at home and never count as opponents for victory. Their level grows with the distance to the core, and every 30 s they gain levels until they match about half the player's army strength.
+- Knight treasure caves only exist where a campaign mission places them; everywhere else dwarf caves took their place.
+- 30% of the dwarf caves hold a **rune chest**: claiming it gives one free build of the **rune forge tower** (1400 HP, range 10, the rune bolt jumps on to three more enemies). It can be upgraded for gold (800 / 1200). Selling it returns the base tower once: in the campaign as a chest for the next mission, in free play at once. Unbuilt rune towers carry over to the next mission.
+- Imps repair damaged towers for free; repairs come before claiming more ground.
 - Savegames store the tiles packed (run-length encoded, base64), so even 256×256 maps stay small (~300 KB); version 1 savegames still load
 
 ## Game content