There doesn't seem to be any reason for this. But everytime we get to this point, my friend's game crashes to the desktop. The error is apparently with his exe file. I have a save file from when he drops, i'll post it below.

If anyone has any insight please help us. We are really enjoying this game and its providing a great opportunity for my friend to learn new strategies.

Thanks.

http://forums.civfanatics.com/uploads/135421/VOTE_BC-0625.CivBeyondSwordSave