During the Build or Adventure Phase one player is the active player relative to the other players. The active player has the advantage of having all Spells and Effects take priority over opponents Spells or Effects.
In the event that two non-active players dispute order of using their Spells or Effects, the player who has the Boss with the highest XP goes first.
In the build phase the player who has the Boss with the highest XP becomes the active player first. The player is the active player until declared they their turn is over, if they choose to build a room this phase that must do so when they are active.
Once all the players have had one active period that turn, the Build Phase is over and the built rooms are revealed.
In the Adventure Phase the active player is decided by whose dungeon currently has a Hero adventuring. Starting with the player with the highest XP, heroes go into Dungeons one at a time in the order that they came into town and while the Hero is in a player's dungeon that player becomes the active player. Once the Hero is out of the Dungeon a new active player is decided based on which player's boss has the next highest XP.