Eternity allows level editors make the middle textures of linedefs into solid objects that can be walked over or under, and can block map object passage. Monsters can walk across them, and they can even be moved (with the assistance of some linedef specials) to create lifts, doors, anything you like. They are quite useful and easy to implement.
This feature is called 3DMidTex, and this tutorial will explain how to use it.
Note: this is obviously unfinished. Check back later.