I was just looking around in the Battle Map Editor when I saw this weird thing.
I was able to get different types of forts.

This is the usual fort in game:
Spoiler Alert, click show to read: 


This is one of the upgrades:
Spoiler Alert, click show to read: 


And this is the second upgrade:
Spoiler Alert, click show to read: 


So is there anyway to mod the game so that the forts could be upgradeable?