I absolutely agree that the fact that cities are nearly impossible to take pre-classical era is a great thing. Ancient era is/should be about defending your civ from barbarians and the occasional hostile raid/worker steal/pillage from those crazed Aztecs (which, unfortunately, seem to have lost all their 'bite' in Civ5?).
The problem is that it goes beyond this. Following the patch, cities with walls and especially castles heal at an absolutely ludicrous rate, not to mention they inflict a healthy amount of damage now. Even this is somewhat OK, however, because after all, it means the city did spend production on getting up a defense. Still, castled cities require something like three contemporary siege machines to break down because even if they inflict a lot of damage, the city heals almost nearly as fast on the following turn. Even with three trebuchets, expect to take a considerable amount of rounds wearing down the defenses. All the while, the besieged city can easily defend itself by putting just a single siege machine of their own comfortably within the protection of the city hexagon - it will deal so much damage it can practically kill off one of the invading units coupled with the city's own attack each turn. It's a bit much, to put it gently. While the defender should definitely be favored in combat, it is perhaps slightly overboard.
What I've been trying to get at all the time, however, is rather how cities INSTANTLY have a fully functional defense from the very moment they are settled. That is just not right, and it opens the door on so many cheesy things: Running a protected settler up to enemy borders and slapping down a settlement while rushbuying a 'walls', BAM, instant frontline fortress. There should be a penalty time to represent the 'Settler' turning into an actual 'city' which didn't even exist moments before. Once again, I believe cities should be founded with health 'in the red' to prevent this from happening - and rushbuying anything should be disabled for the first 5-10 turns, standard speed.