I call the next barrier a "millstone", because I'm not sure what else to call it. It's a big round stone that can be rolled in front of a doorway. I had this idea maybe a year ago, after watching an episode of "Cities of the Underworld" which showed an underground complex in, I think, Anatolia, which used something exactly like this. The idea is that intruders outside the room where the millstone is located can't move the millstone, because there's nothing for them to grip and it's too heavy to push. Of course, a couple ogres in a dungeon might be able to topple it over. Might. The stone is in a groove, which makes it even harder to budge unless you can stand perpendicular to the stone's rim and push it. A couple ordinary human defenders, or four goblins, can do that.
Those who examine the portcullis geomorphs carefully will realize that I scaled the millstone image down to make the wheel for raising the portcullis. Which suggests that, although it's called a millstone, you can change it to some other material, such as wood, if that seems more appropriate.