Gloom Keep is the final part of a quadrilogy of medieval levels in the first episode of Quake, succeeding The Castle of the Damned, The Necropolis and The Grisly Grotto. All four levels are from the mind of author Tim Willits, lead designer at id Software.

There are no monsters in the keep that haven't appeared in the previous levels, but new armaments are available in the form of the super nailgun and rocket launcher. The keep is well defended not only by hordes of knights, fiends and ogres, but by more terrifying traps including large moving spikes built into the walls and automated flechette launchers in the ceilings of its dark hallways.

As with the previous two levels your progress here is hampered by locked doorways to the inner sanctums of the fortress and your primary objective is to find the keys to allow yourself to move forwards. The gold key sits in the centre of a large room filled with fiends and a Shambler, but a secret teleporter allows you to collect a Quad Damage from the ramparts before entering there, to even the odds.

The exit to the level is below ground near the central hall, but you must take an elevator up before you can reach it. There's a steep drop to jump and then it's onto The Door to Chthon, the penultimate level of the episode that sets the tone for your final battle with Chthon, a fiery demon who dwells in a huge pit of lava.

Gloom Keep is one of the most well known Quake single-player levels, and was an inspiration for many other great custom levels to come, including the deathmatch classic Blood Run, the only custom level to date to have been ported to Quake Live.

A cut down version, E1M5 Duel, was created by Oliver Desmet, with much of the deeper sections of the keep removed to form a simple rocket-arena style battleground based around its central hall. A Capture the Flag version of the level was created as IDCTF1, which appears in the Threewave level set as CTF2M7. The structure of the level doesn't work that well as a deathmatch arena but lends itself perfectly to Capture the Flag, and it is well worth checking out.