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.