Do you have a screenshot of succesful use of them in multiplayer ...
Check thommys game against core min 59+
or builders game against Theofanous min 35+
Yes fortress, I think we all know how to use a GG. I can't believe tommys opponent used his GG first to build an undefended citadel. I did learn somthing though you can't plant one on oasis as Tommy frantically found out. I saw their opening moves of that vid b4. Pretty cool cutthroat. Got to start playing mp I guess. Although it seems it's all a CB slugfest.
Fortresses are only built in two circumstances:
1. playing tall in the mid/late game as insurance on your borders, past where you can access the tiles anyways.
2. forming a circle around citadels to create a nearly impenetrable defense.
Doesn't apply for enemy units!
And here is the image:
Oh, I should have added - I don't actually build the things, just start them for a turn to destroy the wheat. Then they carry straight on spamming trading posts.
Well, I lost all my civ files! But um, the Greek player only has archers and warriors, so he's not so good!