I have played your game a bit more, and so far we always kept one monster alive to be able to build our base. So, maybe you should make the game more like we are playing it.

Like, you could make a mode in which there won't be any waves for a long time, or where we have to trigger the wave. To prevent the player from making infinitely big bases, just put a number of crates there and don't let any new crates fall down. This way, this game is about how to best use the available crates.

(Also, a fun mode would be cool where you can build a huge base from a top view )