Each city is split into different City Zones. Possible zones include Airspace, Outer walls, Tunnels and Garrison. Not all cities have all types of city zones, and some may have some that aren't listed. However, each city must have a Garrison City Zone. Furthermore, each City Zone may be split into different sub-zones.
Airspace can only be entered by Flying units. Airspace can attack, or be attacked by, Tower garrison sub-zone.
Outer Walls can contain a Gate subzone. Gates, while weaker than other sections of the wall, allow non-flying, non-tunnel capable units to enter and exit a city.
Outer walls can also contain towers that boost it's defensive value. It has yet to be seen if outer wall towers are a true sub-zone or an enhancement providing addition.
Tunnels can only be entered by light Units or units with a natural ability allowing them to enter tunnels, like Spidews and Sourmanders. Tunnels can attack, or be attacked by, the dungeon garrison sub-zone.
(For a description of the Unit attribute Garrison, see Garrison Ability.)
All cities have a garrison zone. When the garrison falls, so does the city. Garrisons can contain sub-zones.
Each sub-zone could be attacked from only one of the main city zones. Outer Walls only connect to Courtyard, Airspace connects to Tower, and Tunnels connect to Dungeon. Once an attacker enters a Garrison zone, it can assault any other garrison sub-zone.Erf-b1-p116a
Each city may have different Garrison sub-zones, depending on it's construction. For instance, a sub-terrain city wouldn't have an airspace and therefore, no tower. Similarly, not each garrison sub zone needs to connect to main city zone. A city may have a dungeon sub-zone but no tunnels.