In the main selection screen, highlight any title and press R1. The title will flip over and a code will be revealed. Return to the main screen and enter the revealed code to see a bonus video clip.

When your just up to the end where the gate blocking the door is go to the left and jump on the blocks next to the sea. Walk north and you'll enter a secret tunnel with one zombie and a special power-up. Now try getting 100% weapons and and you'll get a message