Eternity provides the possibility of using the middle textures of linedefs as 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.
For the purposes of this tutorial, I'll assume you are familure with Doom editing and your level editor. I use DoomBuilder, but whatever you use should be fine. First, open up a new Doom2 level for editing, create a simple room with a pit running through it.
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Note: this is obviously unfinished. Check back later.