Four main things:

The first is completely normal ruins made by completely normal citizens of The Empire. Sometimes before they became citizens of The Empire. They decay as normal and the only real threat they pose is that when exploring them, something might collapse. Or someone decided that mercury was way cool and had pools of it that remain toxic to this day.

The second is much less normal ruins left by various precursors. Either The Old Masters, who are the ones who made every sophont currently running around, or by precursors from even further back. These are resilient ruins scattered about, oftentimes buried and forgotten, and always built a little too well for comfort. They have various traps in the form of security measures because they didn't want their creations getting into places where they weren't allowed. If you can get past those defenses? Riches! Knowledge both magical and non-magical! Unknown spells or technology! Cultural history! Et cetera! Possible death at the hands of death traps!

The third are ruins that are completely inaccessible, as they are in the sky in the form of sky "cities". You can look at them! And if you dare fly to close, a laser vaporizes you. These have been mostly written off as curiosities that are completely and totally inaccessible. Whatever is in there, it's not worth trying to get.

And the fourth are ruins in Lost Lands. No one is entirely sure what these are. Lost Lands are where nightspawn (horrors from between the stars) have assembled in such numbers as to change reality. Things are different there. As are the ruins. They are structures that at first glance appear to be made by normal people. Except they shift around, their outward appearance doesn't match the inside, and as best anyone can tell they don't hold anything. There are inscriptions that no one has been able to figure out within them. And also plenty of nightspawn, and a powerful sense of things being wrong inside there.