Doom/E3M1: Hell Keep

Mission 8: Dis
If you collected the BFG, rush the Spider demon with that weapon - it will fall in only two hits.

Otherwise, you will want to run to the left and collect the rockets and cells. While you can defeat it with the just the weapons and ammo available on the map, there is almost no room for error.

Once you collected the first batch of rockets, run to the center and open the door. Pickup the Plasma gun and blue armor, and destroy the spider demon.

Secrets
There are no secrets on the map, not counting the middle area.

Deathmatch
This map isn't designed for deathmatch. There are only two weapons, and a single blue armor in the middle.