- This level occupies the map slot MAP11. For other maps which occupy this slot, see Category:MAP11.
Map11: Echidna is the final map in Harmony.
- Proceed north.
- Fight the three Phages, and use the lift to enter the castle. Make a right turn, but watch for the enemies attacking from the north and south windows. A door will open as you approach it, leading to the eastern wing.
- Enter the eastern wing, fighting against followers, and collect the green key.
- With the blue key, re-enter the building by the alternate entrance. Continue north, fighting against any enemies in you way until you reach a switch. Flip it to lift a grate found in the slime, which you can follow.
- You should appear in the room, with a grenade launcher visible. However, going past the weapon will release the centaur. If you can't kill it, the best way is to circle behind the throne on the left, flip the switch, and continue the loop before taking the north-west exit. There is a phage in they way - kill it quickly.
- Follow the slime path on the shoulders, until it branches west. You now need the yellow key. At the west end of the map, you can find it, but a centaur is standing on top of it.
- Open the yellow key door, and go through the darkened area. Climb the rocks, and drop down.
- Head north, and right. Collect the purple key, and return. Head north and open the purple key door.
- Continuing north leads to a health and ammo resupply. To proceed to the Echidna, press the switch to the north east. Use shells until you drop into the boss arena.
- The Echidna fires two homing projectiles, and if you are still in line of sight after they fire, follows with minigun fire.
- At walkthrough step 2, a door will open if you progress. To the right of that door is a secret door that may be opened, leading to 200% armor.
- At walkthrough step 7, you will drop down. To the left is a wall with a misaligned texture. Use it, and you get a backpack and medikit, along with a route to backtrack to an earlier point in the level.
While not credited as a secret, you can find an easter-egg room in Echidna's lair. After defeating her, search the south wall for a hidden door.
- Two linedefs (283, 2713, both covering exits from the entrance) have a tag of 15. They are supposed to start moving a floor up and down, but no such floor exists.