From GEWar Manual
Whenever a city becomes unoccupied, the citizens form a one army militia to fight off any incoming armies. This happens even when you attack and win a city and already leave again shortly after. The militia re-forms 'instantly'.
In order to take the city, you must first attack and destroy that single militia army. The size of that army (the number of troops it contains) depends on the value of the city and how long it was left unoccupied.
All cities have a minimum militia of 100 troops.
1 trooper is added to a city for every 100k of 'value'. Thus
- Diego Garcia. (Value 3,500) = 100 troops (min default).
- Coffs Harbor. (Value 48,047) = 100 troops (min default).
- Banda Seri Bagawan. (Value 67,077) = 101 troops (value rounded up)
- Guangzhou (Value 9,940,000) = 100 + 99 troops = 199 troops.
The longer a city is unoccupied, the larger a militia will be. Unoccupied cities gain one trooper per day. Over time this will grow to a maximum value of 2,000 troops, depending on the value of the city. Also note that militia gains experience through won battles in the same way players' armies do.