A Certain number of moves, or, give the player objectives they need to stop enemies from destroying. This could be interesting giving each enemy piece a goal of either seeking out the player, or one of the objectives the player needs to defend.
Viewing post in Rook's Last Stand jam comments
This was actually my first idea! I had to scrap it unfortunately because of scope creep issues and failure to imagine the final product. The enemies actually have a hard-coded aggressiveness value that prioritizes the rook more than the center, but in the final version every enemy has it set to maximum so it ignores the center and moves towards you always