Well Im not sure if thats actually the reason, so maybe yall can test it and see if you get the same results.
To recreate the CTD:
1) load 'error' savegame and click 'end turn', after the AI finishes moving the game will CTD. Ive already destroyed the fort in question leaving a STACK of AI boats on a plains tile. I have 'see enemy moves' on so you can see that the stack actually does move off the tile, thats why Im not 100% sure its the reason for the crash.
2) load 'pre-error' savegame to see that you can bypass this CTD by not destroying the fort which you can see in my screenshot.
PS: the fort was destroyed by my only 2 bombers in the city south of the fort.
http://shredz2001.homestead.com/files/SS/Error.CivBeyondSwordSave
http://shredz2001.homestead.com/files/SS/Pre-Error.CivBeyondSwordSave
To recreate the CTD:
1) load 'error' savegame and click 'end turn', after the AI finishes moving the game will CTD. Ive already destroyed the fort in question leaving a STACK of AI boats on a plains tile. I have 'see enemy moves' on so you can see that the stack actually does move off the tile, thats why Im not 100% sure its the reason for the crash.
2) load 'pre-error' savegame to see that you can bypass this CTD by not destroying the fort which you can see in my screenshot.
PS: the fort was destroyed by my only 2 bombers in the city south of the fort.

http://shredz2001.homestead.com/files/SS/Error.CivBeyondSwordSave
http://shredz2001.homestead.com/files/SS/Pre-Error.CivBeyondSwordSave