Pick a staircase in this room and go up it, then jump to the teleporter.
Go right outside this teleporter and travel north until you reach a building with many enemies on top. There is also a large teleporter surrounded by barrels outside of the building. It is wise to kill as many enemies on top of this structure as you can before teleporting inside of it.
Head to the central building, using one of two paths:
Go through the large teleporter on the left, and use caution getting through this area. Go up the elevator at the end of the path.
At the left, there is a switch. Hit this switch, and your business in this building is over.
Just southeast of the building, a bridge has now risen allowing you to cross the moat. Cross the bridge.
As an alternative to the above few steps, jump into the pit, and enter either teleporter from the right-hand edge.
You should now be on the central platform, and need to reach the column on the southern part of the island containing an invulnerability on the top.
There is a switch on the south side of this block. Hit it, but instead of going through the teleporter that drops down next to it, go around to the north side of the block and up an elevator that drops down. This will give you the invulnerability that you may need for the next part.
After teleporting, you will be in the prison room. To break free, the four corners must be opened and switches inside them must be pressed. This must be done quickly because each switch opens the door to the next switch in its corner. At the last corner, there is a red key and a teleporter. Get the key and enter the teleporter.
Go through the red door in front of you. Kill everyone. (Including the revenant hiding in the unaccessible exit room.)
At the south wall in the lava pit there is a radiation suit. Jump down and get it, and hit the button along the south wall. The pillar in front of you can be used as an elevator to get you back to the safe floor.
The button opened a new area of enemies, and a teleporter. Travel through the teleporter, jump across the pillars to the exit door, and press the exit button.
In the first area are two stairways leading up; take either one. When you approach the top, the wall in front of you starts to lower. Wait until they all lower completely. Try to cross over to the teleporter from the highest portion of the stairs, and the floors will lower a little more. Drop down to the right. There is a patch of wall to left that doesn't match the surrounding texture. Open it and flip the switch. Now make your way to the opposite side of the raised teleporter platform. A secret blue alcove containing a berserk pack will have opened.
At the west end of the outdoor area is a raised tower. To get it, take the large teleporter in front of it. At the top is a raised platform with a soul sphere on top in a small room, which is surrounded by a balcony. As you walk along the balcony, you'll hear a floor lower. Run back into the soul sphere room, where the powerup has been temporarily lowered. Pick it up to register the secret.
On the "island" in the middle of the lava lake is a gray brick cube with a switch on its back side. Flip the switch, but instead of entering the teleporter it lowers, run around to the opposite site of the cube, where another lift has lowered, this one letting you reach the invulnerability sphere on top.
In the area at the east side of the map is a dark room with six columns in a lava pit. The center column of the northern row (the one with lights at its base) has a broken pipe on its northern side. Press use on the broken pipe, and it will open a secret room at the very east end of the room, across the bridge. There is another soul sphere inside.
If the map is played either on skill 1-3 (because there is enough room for the player and the bad guy that is down there on those skills) or in a source port that makes actors not infinitely tall, it is possible to get stuck on this map by falling into the demon-containing triangles in the north west corner of the map if they haven't risen (this can be achieved by using the south sides of the teleporters in the lava in front of the red door, as they teleport you to the north where the triangles can be reached without crossing a trigger that raises them).
Speedrunners may want to dive into the damaging floor and enter the teleporter from the right-hand edge. This allows you to skip raising the bridge.
It is also possible to jump to the island from the south using SR50.