A central arena (more action on the ledges than the floor, at least for us), and some outside narrow passageways. Author says powerups and rechargers were kept to a minimum... I guess we have different definitions of "minimum". (There are 2 2x rechargers, and a 3x powerup, that I found...) Teleporters in the outside corridors make sure it's never hard to find spanker-fodder...