Do not open this yet instead, destroy every Revenant and Baron of Hell that exist in the room, then proceed to open the north security door. Now go to the door west from the starting room, walk across two sets of stairs and you'll arrive in a room with two locked security doors (one north and one west), a cage protecting the red keycard and a door which requires the blue keycard. You'll be warped to the tall pillar outside, allowing to grab the blue keycard. Kill him, wait until the teleporter lowers completely and step onto it.
Meanwhile, an Arch-Vile will spawn from the green marble demonic square on the floor. Inside, flip the switch to lower a teleporter atop a tall column on the southwest corner of the 'house'. This will open the doors of the 'house', which will close and lock once you've entered. Kill all monsters here, then climb up the eastern 'stairs', dispose of the Zombieman and hit the switch.
This place is full of other Shotgunners, Arachnotrons, Pain Elementals and Lost Souls, along with a confined 'house'-like structure and a tall pillar where the blue keycard rests. Turn to your right to see a huge (but harmless) lava 'lake' with 2 access 'stairs', one west and one east. From the starting room, open the east door, kill the Shotgunners/ Zombiemen, Demons and Spectres, and head all the way to the end of the hall.
