Extra multi-player map:
Reach the Pier Pressure bonus round (after Year Of The Pac) and get a score of 50,000 or more. A message at the results screen will confirm that a new map has been unlocked. Save the game if you have a memory pak (if not, select "Quit Game"). Quit the game and go to "Multiplayer" option on the main menu. You should have a new place to battle a friend.