MAP15: The Theory of Broken Circles (BTSX-E2)


Back to Saturn X E2 maps 12-16

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(12 - 16)
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Secret map: 31

This level occupies the map slot MAP15. For other maps which occupy this slot, see Category:MAP15.

MAP15: The Theory of Broken Circles is the fifteenth map of the second episode of Back to Saturn X. It was designed by Björn Ostmann (Vader) and uses the music track "Geometry," composed by James Paddock (Jimmy). In the official add-on version, this level's name is Birdwatching in the Forbidden Realm.

The secret level, MAP31: Fireking Says No Cheating, is accessible from this level.


Map of The Theory of Broken Circles
Letters in italics refer to marked spots on the map. Sector, thing, and linedef numbers in boldface are secrets which count toward the end-of-level tally.


The keys in this level may be collected in any order.

Yellow key[edit]

Step forward to collect a chaingun and kill the imps overlooking it, then activate the lift in the north-east corner and ride it up to see some more imps. Turn right and go through the wide doorway ahead to step out on to a gray walkway, then turn right again and follow the walkway to a super shotgun (guarded by Hell knights on Hurt Me Plenty (HMP) skill level, or a baron of Hell and two mancubi on Ultra-Violence and Nightmare). Step across the square platforms here to another lift and ride it up to a large chamber with imps and/or revenants; you need to straferun across the square platforms ahead to reach a lift that goes up to a flame wall teleporter, but the surrounding monsters can knock you off with their projectiles, and if you kill them then they will be resurrected by hidden arch-viles.

Drop into the blood below then head up the steps in the north-east corner to confront a cacodemon. Turn right and right again to step on to a stone path and follow it round to a plasma gun, then turn right to see a diamond button that will crush the arch-viles and stop them raising the monsters below. Kill one or two arch-viles that have teleported into the blood river behind you, then go back round to the lift leading up to the chamber; you can now run across the platforms to reach the flame wall teleporter, which sends you to the yellow skull key in the starting room.

Blue key[edit]

From the starting point, activate the north-east lift again and ride it up, then turn left and follow the path until you leap over a gap to a megaarmor; do not stop moving as a cyberdemon is waiting on the path ahead. Follow the trail of armor and health bonuses to either of the doorways in the north and east walls and step through to an enclosed chamber with two imps or revenants; the floor will rise and you will have to contend with spectres, imps and revenants that teleport in as well as other enemies that get in from outside. Go through the north-east doorway to see a flame wall blocked by bars - there is a diamond button next to it that will open some of the bars.

Head outside and drop into the blood river below; you will need to follow the river round to a pool north-east of the cyberdemon's room, where you will see two lifts surrounded by candles. Use either lift to go up to the chamber you did not enter earlier and kill more enemies that teleport in, then go through the north-east doorway again and press a second diamond button to open the remaining bars and allow you to enter the flame wall, which will send you to the blue key in the center of the cyberdemons' room (there is also a BFG9000 on HMP or higher). One or two more cyberdemons will have teleported in, so quickly step off the platform to teleport back to the outer path.

Red key[edit]

From the starting room, ride the north-east lift up and turn right, but this time follow the hallway round until you reach the west side, where you can step over two small square platforms to a circular walkway with one or more knights. Run off the outcrop on the west side (which holds a supercharge on HMP or lower) to land in a marble hallway, then turn left or right and follow the path round to find one or more revenants guarding a super shotgun and a diamond button. Press the button to open a flame wall behind you, kill any imps and revenants that teleport in (plus one or two arch-viles on HMP or higher) then step through the wall to teleport to the red skull key. Do not loiter on the platform as one or two arch-viles have been released to the west.


With all three keys, go through the north-west doorway in the starting room and head north up some steps to find a mancubus guarding a pair of small platforms, which you can step across to find a staircase leading up to two lifts. Activate the right-hand lift and ride it up, kill any mancubi waiting inside the building to your left along with an arch-vile that teleports in (use the rocket launcher here to help you), then step on the tiled lift to go down to a room with revenants, a baron and two demons (or a mancubus on HMP or higher). Open all three sets of colored bars and press the buttons behind them to raise the platforms inside the blood pit, then run across the platforms to reach the exit teleporter to MAP16: Tower in the Fountain of Sparks III.

Secret exit[edit]

To reach the secret exit to MAP31: Fireking Says No Cheating, you must have the blue and red keys and have stepped on the floorplates in secrets #3, #8 and #9. Once this is done, follow the directions below to secret #10 and go up the steps to a mural (which shows the locations of the plates you stepped on), then step across the platforms opposite the mural to a lift and ride it up to a walled path. Take the supercharge ahead and turn right to kill a mancubus, then step across the platform behind it to some colored bars (noting one or two more mancubi that teleport in behind you) and open them with the keys you have obtained before riding a lift down to a chamber with various enemies. Step across the square platforms to reach the other side, then press the buttons at the north and south end of the chamber to open the flame wall at the bottom of the steps along with two monster closets holding revenants. Step through the flame wall to go to the secret level.

Other points of interest[edit]


  1. From the map start, take the lift to the right. The wall to the left has a lighter patch in the middle. Press on it to lower it, revealing a passage (sector 140) where you can drop down on a supercharge.
  2. In the green stone hall by the red key, the southern alcove with a green torch behind an inverted cross window hides a crucifix switch on the rock wall. Shoot it to open a wall north of the entrance leading to a platform (sector 490) with ten health bonuses and a computer area map.
  3. On the south-southwest edge of the map, there is a narrow cliff with a bloodfall on the south side. Go through this bloodfall (sector 500) to find two boxes of rockets, a partial invisibility and a floorplate.
  4. On the very west end of the map, in the C-shaped stone hall, a wall at the bottom end of the C is lighter than the rest. Open it to access ten armor bonuses, an energy cell pack and a box of rockets. (sector 552)
  5. On the blood basin west of the C-shaped hall, run up the rock pillars to the north to reach the plasma gun on the island (the beige pillar lowers permanently; should you fail to make the crossing or fall off the island, there is a teleporter in the hallway to the north that takes you back to it). When you jump to it, an alcove with revenants will open. Inside this alcove is a cross switch behind the green torch; shoot it and take the teleporter nearby. Lower the wall with the lift-like texture to reveal a small closet with a tiny passage to a teleporter that takes you to a high ledge (sector 700) with a supercharge.
  6. Near the north-west part of the map, on the ledge with the shotgun, hop to the black stone pillar to the south and then to the wall directly west of it. You'll land on a narrow, almost imperceptible staircase that runs along the wall. Climb to the top of it and a pillar will rise out of the blood. Hop on it as it rises (it can be lowered like a lift if you fail the jump) and when it reaches the top, hop off it and onto a ledge (sector 842) with nine health bonuses. From here, you can reach the opposite cliff to collect a megaarmor and energy cells.
  7. On the arena with the resurrecting revenants at the south end of the map, drop down to the blood below and up the stairs to the east. As soon as you climb the first step, you'll hear something open. Sprint up the stairs, onto the blood canal and through the bloodfall beside the hole in the wall to reach a hidden area (sector 1022) with two rockets and a megasphere.
  8. From secret #7, press on the grey wall with the face to lower it, revealing a circular room (sector 1382) with two rockets, a backpack and a floorplate.
  9. Stand in front of the blue key bars and turn right. Follow the hallway to a lift, and when it rises, stand on the west half of it. Once it reaches the top, run out of it; as it lowers, another smaller lift to the left will also lower. Ride it down to a small circular room (sector 1423) with two revenants, two stimpacks, three shotgun shells, and a floorplate.
  10. There are three teleporters in the map that cycle among themselves: one on the south-west near secret #3, one to the north-west by secret #5, and one to the south-east. From the south-east teleport, turn right and enter the dark staircase. At the top, go through the bloodfall, then turn around and head through the other bloodfall (sector 1395). This leads to a cavern with a stone mural depicting the map, with three cutouts indicating where the floorplates in secrets #3, #8 and #9 are (activated ones are dark, untouched ones pulsate green). After all the three floorplates are activated, the two pillars in the blood pit will be tall enough for you to cross over to the lift opposite the mural to the secret exit.
  11. From the lift mentioned in secret #10, cross the stone walkway and drop down to the blood when you can. Head north-west and through a short opening to find a pillar (sector 1253) with an energy cell, an energy cell pack and a BFG9000.


The gray blocks south of the button behind the teleporter to the yellow key (linedefs 4182 and 4209) do not have textures on their north faces, resulting in a noticeable hall of mirrors effect.

Demo files[edit]

Areas / screenshots[edit]


Routes and tricks[edit]

Current records[edit]

The records for the map at the Doom Speed Demo Archive are:

Run Time Player Date File Notes
UV speed (normal exit)
UV speed (secret exit)
NM speed (normal exit)
NM speed (secret exit)
UV max
NM 100S
UV -fast
UV -respawn
UV Tyson
UV pacifist (normal exit)
UV pacifist (secret exit)

The (absence of) data was last verified in its entirety on February 21, 2022.


Player spawns[edit]

This level contains thirteen spawn points:

  1. facing north. (thing 5)
  2. facing south. (thing 1293)
  3. facing south. (thing 1294)
  4. facing east. (thing 1295)
  5. facing east. (thing 1296)
  6. facing north-west. (thing 1297)
  7. facing north. (thing 1298)
  8. facing west. (thing 1299)
  9. facing east. (thing 1300)
  10. facing south. (thing 1301)
  11. facing south. (thing 1302)
  12. facing south. (thing 1303)
  13. facing north. (thing 1304)


Map data[edit]

Things 1614
Vertices 6896*
Linedefs 7059
Sidedefs 11096
Sectors 1493
* The vertex count without the effect of node building is 5470.


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

Technical information[edit]

Inspiration and development[edit]


See also[edit]


External links[edit]