Only some of the room cards say "Destory this room (and other stuff happens)" on them. But when can I destroy a room? Does it have to be on my turn, or can I do it when heroes are in walking through another player's dungeon?
I took a deeper dive into the full rules and some Room cards, and uncovered some relevant snippets.
Under "Build Phase":
Once each player has had the chance to place a Room during the end of the Build Phase ... "when you build this Room" abilities take effect.
Under "Sequence of Play":
At the End of the Build phase, any Level Up or "when you build this room" effects trigger...
Under "Sequence of Play, cont'd" > "Rooms Deal Damage":
As the Hero enters each room, any Room ability takes effect, then the Room deals its damage.
You many only destroy a Room if it is active, and if a card specifically allows you to do so.
Zombie Prison (Room):
Destroy this room: Choose a dead Hero in an opponent's scorekeeping area. Send it back to the entrance of that player's dungeon.
The Crushinator (Room):
Destroy another Room in your Dungeon: Until end of turn, your Rooms have +2 damage.
Jackpot Stash (Room):
Destroy this room: Double the treasure value of your dungeon's Rooms until end of turn.
Bottomless Pit (Room):
Destroy this Room: Kill a Hero in this Room.
- You can't destroy rooms at any time; only if some card allows it.
- If room destruction is time dependent (such as during the Adventure Phase), you can destroy the room at that time. (This is because of the "Room abilty takes effect" clause under "Rooms Deal Damage".)
- Otherwise, Rooms must be destroyed at End of the Build phase so that their "Until the end of this turn" effects apply.
This "Detailed Play Sequence" helps clarify this.
Destroying a room isn't something you can just "do". During the build and adventure phases you can take actions either through playing a spell or using a room's ability, and the cost for those actions might involve destroying a room. This means that any time you can take an action, you might end up destroying a room. This "Detailed Play Sequence" helps clarify this.
You may play a Spell or Room Ability when another player is active, but they can react with their own Spells and Room Abilities, and those resolve before your effect.
The Build phase ends when Rooms are revealed. Build effects ("When you build this Room...") and Level Up powers take place immediately, in XP order. No Spells or activated abilities may be played until the Adventure Phase begins.
Actions can be used on any player's turn during the build or adventure phases. If multiple players want to take an action at the same time, the current player always has their actions take priority, and then the rest are resolved in XP order.
In the build phase, you generally take actions when it is your turn to select a room. At this point you are the current player, so nobody else can interrupt your action (except to counter it). You are not allowed to use build phase actions after rooms are revealed, so you can't play a room and then immediately destroy it before the bait phase. The only thing that happens after rooms are revealed is "Level Up" and "When you build this room" effects, followed immediately by the bait phase.
In the adventure phase, each time the heroes move to a different room, there is an opportunity for any player to use an action. The player that owns the dungeon the heroes are in is the current player, so their actions will have priority.