B21

B21 is a single-level PWAD for Doom that was released in 1994. It was designed by Bob Carter. It was later adapted as E3M6 of The Lost Episodes of Doom.

Secrets

 * 1) In the area after the start is a round platform containing a soul sphere.  This can be obtained by running onto it, or by pressing any of its walls on the north side. (sector 16)
 * 2) The south-east of the trench near #1 is narrower than the rest – walking over this part will open up a door behind the start, leading to a room with a chaingun.  It will also open up the northern part of the trench. (sector 3)
 * 3) The north-west contains a room with various floors going up and down.  There is a room to the east of this with a switch, which opens a room to the east of the southern oscillating floors, containing a blue key, a spectre, an energy cell pack, a medikit, a rocket launcher, and a rocket. (sector 72)
 * 4) The switch that opens up #3 also opens a room off the middle-right of the fifteen northern oscillating floors, which contains a switch opening up an area north of the easternmost of the three crushing ceilings.  This leads to a BFG9000, a partial invisibility, a health bonus, and some ammo.(sector 227)

Current records
The records for the map at the Doomed Speed Demos Archive are:

Things
This level contains the following numbers of things per skill level: