I don't see anything out of character about a General being associated with the construction of a base or fort. Admirals may oversee the building of ships, but generally not forts (even coastal), which are usually under the auspices of the Army, not the Navy.
Well, not wanting to drag no one into flame, but I'll repeat my already stated point, that establishing a naval fortresses was a common practice for Navy's. Especially in farest territories, during conquering/exploration where there was obviously no ground forces to help them build such improvements.
I think, the main complexity in giving GA such an ability is in game mechanics, together with questionable necessity of it.
-Necessary? -Not really.
-Diversible? -Yes!