Undead! My favorite adversaries; well at least when using them against the characters
I think undead can be:
ethereal: meaning they exist in the Plane of Shadow, or world design analog. They are not hampered by any non magical construction on the normal plane of existence. Ghosts and wraiths are an example of that.
Energy undead: they consist of energy and bound by those rules, so they could pass through metalic object, or crystalline objects and even the tiniest crack would allow them to pass. Spectres and fire phantoms fall in this category.
Gaseous undead: Somehow they have bodies composed of gas, perhaps because they were vaporized, or because they were tied to gas somehow. They can pass through tiny cracks in stone or around windows. (I like roofs best since most are loosely constructed). Gaseous vampires (yeah I know they don't have that ability in RM) or Dirges fall in that category.
Lastly: the solid particles or fluid undead: smoke, blood, or sand are not incorporeal by definition, but still act in certain incorporeal ways. I allow them to pass cracks 1/4 inch wide. Desert spirits would fall in this category.
All of the above would be stopped by wards and circles of protection of course and usually some energy barrier will do as well: such as a wall of fire or a big bonfire.
After a little research into C&T I learned that most undead there are of the ethereal kind, usually with some body part being less ethereal and able to wield tools, or wear cloaks and armor.
Game On!