In the earlier series, the exterior the Carrie's apartment was another apartment block (in one episode we see a couple having sex through this windows). In later series, the exterior changes to the street outside and the other apartment seems to have moved. See more
It's a slippery slope. First you're going once a week, and then it's three times a week, and then the next thing you know, you're starting every sentence with, "My shrink says".
My shrink says thats a very common fear.