I'm currently working on a top down shooter inspired by (among others) "Mercenaries: Playground of destruction". 

I want to give players as much freedom as possible when it comes to finding a playstyle. I've already added the possibilities to break windows or blow up walls to gain entry to a room/building. After destroying most of the walls, it makes sense to have the building collapse, which poses a problem.

Since this game uses a top down perspective, the roof is invisible. How can/should I let the player know he needs to leave a certain area before he's squashed by the falling ceiling? I'm thinking about shaking the walls that still stand and/or letting some debris fall from the ceiling using a particle system. Does anybody have any other good ideas?