One remains;
11: The big nuisance in each level of this game is also a shooting mini-game.
Hint 2: In the regular levels, the holes that spawn the individual nuisances can be 'deleted' by placing something in front of it and removing it again. It's probably a glitch.
Hint 3: They are a nuisance because they are rodents, and that's not what you want to have running around these kinds of halls. Guessed by drbunnig - Theme Hospital