Cities should be structured into distinct zones, and not every city should start with all possible zones.
Cities should be "attackable" and "damageable", regardless of the playing pieces inside them. Attacks may specify the city zone in which they are most interested, thought this might be required only in advanced levels of the game. This specification would incur some attack penalty for previously unmapped cities.
If the manufacturing zone is badly damaged, new pieces may not be produced until it is repaired. Similarly, badly damaged airports and seaports may not be used. Cities should be "self-repairing" at some pre-determined rate, so long as residential zones are not totalled. The defender determines allocation of repair resources to the various zones when multiple zones are damaged, with the exception that repair of residential zones to some intermediate status (call it "fair") must be given precedence.
Cities may "grow", i.e., add zones. One residential zone will suffice to support two other zones (e.g., manufacturing and airport). Four zones constitute one unit of land, so that large cities may consume many land units.
Growth occurs through devotion of manufacturing resources - it's an alternative to building playing pieces.
100% - Undamaged
90-99% - Excellent
70-89% - Good
45-69% - Fair
20-44% - Poor
1-19% - Critical
0% - Destroyed / Unrepairable