strange is you can wait 6 -8 turns before you attack (3-4 YEARS)..

but when the fighting starts you have to be done in 20 minutes


I have sometimes reopened a save game to be able to retry the attack, some cities it's quite a long walk from the gates to the square. Also taking you by several towers that fire on you so you have to take them first... which is not hard (cause they are not defended) but a pain in the *ss