Before the first turn begins, each player can build one Room.
To “build” a Room card, place it face-down on the table to the left of your Boss card. The highest-XP player goes first, then each other player places a face-down Room.
Once all players have placed their cards, simultaneously reveal the newly built Rooms. In XP order, resolve any “when you build this” effects.
At this point, Set Up is complete and the first turn begins.