Your cannon is controlled by tilting the iDevice left or right. You have three basic weapons. The laser beam which is used to change the color of the enemy as well as hold an enemy in place.
This allows you to group two or more enemies for easy dispersal. Rockets which are used to destroy Knurlings, and the bombs which are used to clear everything on the screen.
When you being the game you will have a fixed number of rockets in your arsenal as you destroy sets of Knurlings you will gain additional rockets. Kill 1 Knurling and you get nothing. Kill 2 and you get one additional rocket, kill 3 and you get two rockets etc, etc.