Stop me when you’ve heard this one before: a diverse group of thrillseekers meets in a gathering place, planning an excursion to some ancient site rumored to contain gold. They are certain it will contain traps, but also monsters; surely nothing so prosaic as snakes and rats (what would they eat?) but perhaps imps and the walking dead.
I say: it depends on the age of the ruin. Light research indicates that the oldest door in Britain is a thousand years old (http://news.bbc.co.uk/2/hi/uk/4743899.stm) and the globally eldest five thousand years old (http://www.cnn.com/2010/WORLD/europe/10/22/switzerland.ancient.door/). In a desert environment or something, sure, I guess that’s fine, but if your dungeon contains a water feature or moving creatures, I suspect the fixtures will be rather aged.
And that’s a sedentary, solid door. In that same timeline, a skeleton should become dust. A shambling rotting corpse like a zombie should shrivel to bones, becoming a skeleton — a decade doesn’t seem at all impermissible. A ghoul should degrade, through zombiehood into skeleton — at least, assuming they can’t find a snack from time to time. A vampire spawn similarly, down through ghoul as they starve.
I assume wraiths and specters and ghosts operate similarly; a forgotten king is fine, but a forgotten king of a forgotten kingdom? That seems like too firm a grasp, to my mind: rot downwards into a shadow, and depart.
Mummies and liches and wights, however: the whole point is that they’re preserved unnaturally against rot and time and death. Let them unlive forever, so that you know you’re in trouble when your party faces a horde (not of zombies but) of mummies.