Quote Originally Posted by Sinner
You can't build forts adjacent to ech other, you have to have a gap between them. There also appears to be terrain limits: I've had instances where there are no nearby forts or enemy units but the build button has remained greyed out, and the only common factor was that I was in rought terrain.

If it's swampland you can't build a fort. Also if there is a diplomat/spy next to you, you have to chase the dipomat/spy away to build it there.