To finish Wizard City, you have to finish Firecat Alley, Cyclops Lane, and Triton Avenue. Then you have to defeat FoulGaze in a tower in Olde Town. Then the Headmaster will give you a quest to defeat Lord NightShade. After that you go back to the Headmaster and he will give you another quest to talk to Bartleby in RevanWood. He will tell you to go to Krokotopia and from there on you're on your own. Good luck!