You have to destroy the monsters before that happens, and the only way to do this is to dig a hole in a wall, lure the monsters into it and fill it in quickly before they climb out. However, as soon as you get rid of one set of monsters, they're replaced by another lot of a different colour.
It gets faster and faster and more and more fun. The graphics are excellent and the instructions easy to follow. It's also compulsive, making you ask for the notorious "one more go". You could say MONSTERS grows on you.