When you start the game, the default gun is the pistol. You can hit 2 to use the machine gun or you can hit 1 to return to the pistol mode. (3 to use the Speeder) To use the gun, hold the space button or click the space button to whatever amount you want. You have three lives just like the green zombies. But the orange zombies have 6 lives. To pause the game, hit "p". To unpause, hit "p" again. Oh, and use the arrow keys to move!
Credit to @WARFAME 9/10/21- I added a new feature. You can now hit 3 to play on the speeder. I called it that because the gun was super fast.
