Pale Wind. Which is now the name of map08 (rather than "Fortress" as some of you may have noticed from the 0.7.a).
This track was made a while ago, and it's one of the cases in which the soundtrack inspired me on how to make the level rather than it having been written to fit what exists. Same thing happened for the Alexander Brandon tracks.
The Craftsman inspired the water area at the start of map05, while One Calm Day is pretty much responsible for the industrial aesthetic and the elevator in map07.
The same happened for the Methadone Skies tracks, with Exit the Void having added an entire new chunk of segments in Map06.
And I think Breathelast needs no mention with their masterpieces rocking up the arena segments which were pretty much made to fit the music (hell, the corrupt portal battle has the spawns/quakes/corrupt heart explosions suited 1 on 1 to the track).
In any case, enjoy!