Most controls are in the game, but here are a few others.

Middle-Click towers to mute them

Hold space to cheat and get money

Press Z to skip a level (hold Z if enemies are still on-screen)

Press X to completely skip a level (you will have to do this for the final level of the first world. Sorry)

Under no condition are you to press G. Whatever happens is not my fault if you press it.

Press S to save (from any point, even in the middle of a wave) and press L to load a save.