Strife/Alien Ship

The Alien Ship is the craft in which the Entity came to Earth. Warning: once you enter this level, you cannot return to the previous level, unless you turn back immediately; a situation unique to this level and the next. Although by this late stage, there's no earthly reason to turn back anyway.

This ship is probably the "comet" that caused so much havoc. The décor (especially the color scheme) will look very familiar to those who have beaten Episode 1 of Duke Nukem 3D (which came out a year earlier), although of course the textures are different.

Mission G12 / B11: Go for the Entity
Walk into the first room, and the door will seal itself behind you. Be careful not to fire, since as well as wasting ammo you risk awakening four Reavers hidden in the columns, who if they wake will come out to get you... So just grab as many supplies as you can; then press the switch (A) to open the exit door (and, unfortunately, those columns).

Go though to a big cavern — and a major fight (amongst the many enemies here are two Crusaders and an Inquisitor — and that's just on easy difficulty). Be very careful here since although some of the pits are shallow (some even have a few items in them), there are two lethal drops. Don't descend into any pit to forage for supplies unless you do so via a flight of steps, so you can climb out again. Probably best to save before entering this room, and again before doing any exploring.

Through the exit and to the lift; the bottom landing is the way out, but the top landing leads to some medical supplies (and 20 gold). At the bottom, go across the walkway (be careful, there are lethal drops on both sides); to the north (your left) is an area with a few supplies, but not many, and reaching this area involves risking a fatal fall so it's probably not worth the risk. Out through the door at the other end and you have finally reached the ../Entity's Lair/.

Bugs
The map shows a set of four teleporters; but the linedef to open the door to the first one (behind (A)) is after another linedef which permanently closes the door to that room, so it's impossible (without cheating) to go through that teleport and thus open the doors to the other three.