There are reasons not to re-occupy an old ruin.
The simplest is that the ruin is probably not structurally sound any more. To make it safe to live in, you would have to tear it down and rebuild it.
A second reason is that defensive architecture is a moving target, and a centuries-old ruin would be vulnerable to newer forms of attack. Indeed, those forms of attack are almost certainly how the place became a ruin in the first place. For instance, once people figure out that tower keeps are natural chimneys, you're not going to want to move back into one.
Thirdly, ancient cities were tiny, and an ancient fortress probably is tiny also.
It is quite likely, though, that the LOCATION of the ruin is every bit as strategically important as it ever was. There is ample historical precedent for building a new fortress on the site of an old one, using whatever materials from the old one that you can salvage.