I was playing the mod I downloaded from this site - Realistic Starting Nations. Good mod, even though there are too many barbarians. I have a lot of fortresses built, and they look like square castles on the tiles. Then, 1600 AD comes. . . and every fortress changes it's look from the castle-like thing to something flatter and more open appearing. My workers can still build fortresses but not on fortress tiles, and the ones they build look like this new thing, the one that first appeared in 1600 AD. I assume it was just a modernizing of the look of the fortress. BUT I'M NOT SURE. (They were not made obsolete?). I can't find a reference to this in the rules or strategy book. Perhaps I missed it. (?). Clarifications are welcome.