Some of the cities conquered by Gobwin Knob formerly produced types of unit which the leaders of Gobwin Knob don't consider useful under existing circumstances. In these cases, the city is sacked, and Sizemore later rebuilds the city. The rebuilt city then produces the same sorts of unit produced by the city of Gobwin Knob.
Rebuilding a city: A city that has been sacked can be reclaimed by any side or barbarian and rebuilt into a level 1 city, assuming said side or barbarian has the necessary financial resources. From there, it can grow as any city can.