Letters in italics refer to marked spots on the map. Sector numbers in boldface are secrets which count toward the end-of-level tally.
Secrets[]
Once you gain access to the second portal, look at the lanterns on either side to notice that the eastern one is flickering. This lantern can be walked through; do so from the first step of the staircase, and drop down to find a passage leading outside, as well as into another room with a box of shells, a super shotgun, a berserk pack, a blur artifact, and a switch. Flipping the switch reveals several heads of Romero, although none of them can be attacked. This actually consists of four sectors marked as secret. (sector 150)