Buildable Rocket Shield
Musical Easter Egg (find all the secrets)
If your having trouble finding all the secrets then follow this link for clues as to where they are:
Doom Textures - KuriKai, Chilvence, Freelanzer, Reinchard, DaniJ, Envid, Tea Monster
id Software for making Doom
Sometimes the rocket shield will stop working in certain parts on the map.
Instead of having lifts like in doom the map has some steep stairs. If a large group of zombies goes down the stairs at once they can end up on top of each other, and will stand still for a few seconds, after a few seconds the zombies will continue running normally.