Debug Mode

Go to the password screen and put in "QQQQ QQQQ QQQQ QQQQ". The game will say it's incorrect, but if you go to the OPTIONS menu you'll find a new DEBUG entry. In it you'll find God Mode, Level Select, All Weapons and more!

Secret Exit and Level

In the Necropolis Zone after the huge spike comes by you go into the water area. Go straight and turn right at the split, then when you reach land kill the zombie, open the door, then look up and kill the Ogre on the platform, then walk forward turn around look up and kill the other Ogre. Turn around again and walk toward the yellow armor. Shoot the door ahead of you and you will see a warp. Walk into the warp then you will be on a platform. Jump to the platform in front of you, turn right, then shoot the wall to find a long hall and a warp at the end of it. Stay to the side (either side) of the hall because the floor opens through the middle of the hall and there's lava is under it then jump into the warp to go to the secret level.

New uniform color

After you put the code QQQQ QQQQ QQQQ QQQQ, then clear the password. Place the new password S3TC OOLC OLOR S??? and you will see the new color uniform in the 2-player mode.