This level occupies the map slot MAP12. For other maps which occupy this slot, see Category:MAP12.
From the Start, drop down to the north. Take the right (eastern) passage, descend the stairs and ride up the lift in the northeast. On the ledge, go south to the rocket launcher. If not done yet, kill the two imps on the platform west of you. Run from the rocket launcher platform to the west, jumping over the platforms with the super shotgun and the chaingun to enter the opening in the western wall to find a place (sector 18) with an arch-vile, a box of rockets and a soul sphere.
Push the skull switch on the platform where the (hopefully dead) spider mastermind is on to raise the gap in the walkway north of you (from where you jumped on the platform). Return to the walkway and cross the former gap to lower the teleporter east of that platform. (It is also possible to jump on the unlowered teleporter from the platform by running.) This teleporter leads you to the raised area north of the blood pit surrounded by the aforementioned walkway. Here you find a BFG9000 (sector 186), two bulk cells, some chaingunners you better killed before and an arch-vile that started teleporting counter-clockwise between the southern four alcoves as you entered the spider mastermind's platform. (On hard skill levels, there is another arch-vile teleporting between the two alcoves in the northern wall.) Run up the southern stairs to reach the soul sphere on the central platform in the blood pit. Doing so will teleport three mancubi to the walkway.