Enter these codes while the game is paused.

Effect	Code
Both players invincible	C, A, Left, Right, A, B
Full energy for both players	Down, B, Up, B, B, A
Get upgraded weapons for both players	C, Right, A, B, B, Up
One life added for both players	Right, A, Down, C, A, Right
Play Level menu	Down, B, A, C, Down, Right