Just playing my first game of BtS and this is one of the things I really noticed. I had one of my cities pounding an enemy city with culture for ages (about 400 a turn), sent in about 30 culture spies as well. Just wouldn't budge! When my city went Legendary it eventually surrounded the enemy city, but still it wouldn't flip. Thing is, they had about 25 units in there, which I think helps defend against this sort of thing. This was only on Chieftain, though, so lord knows what it's like higher up.