This level is somewhat similar to the PlayStation port of Doom, with complex, winding corridors, closed-off areas, a river of acid separates parts of the base off and a teleportation network allowing the player to access various parts of the map. You will mostly encounter low level demons, such as Zombiemen, Imps, Demons, Cacodemons, Hell Knights with the odd Baron of Hell thrown in for good measure.
After entering the Blue Door, check the wall behind the floor lamp to your left. An alcove opens containing armor bonuses.
After heading down into the room, with the yellow key, check the right hand side wall as you go in, a corridor opens leading into the closed off area containing the Soulsphere, that was visible from secret #1.