I have noticed, the last few games that I have played, that a tower that shares some of the same area with a city will lose that shared area if the city is razed. I have even had some towers entirely inside the ZoC of a city, and when the city was razed, the tower controlled the square it was sitting in and nothing else. However, the problem is "fixed" if I save the game and reload. I just don't want to have to save/reload every time I take a city, which I can usually manage every turn or three, once I start going on the offensive.