Tower is a Zone of the Garrison Zone of Gobwin Knob. It comprises the Tower of Efdup.
Tower can only be attacked from Airspace, which can only be entered by Flying Units. Tower can be attacked from, or can attack, any of the other two Zones of Garrison -- Dungeon and Courtyard.
Tower may be a zone in other cities as well. Each city may have unique rules based on its own construction.
Chocula was a level two city and Caesar stated that it had a tower. He also said Carpool's Shockmancer had been to the tower and spelled it up hard to take down any air attackers.