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 comelpletely skip a level

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.