This is very interesting! Truly a deadly dungeon and your level assessment seems fair.
I like how you laid out the lore about the place, and I especially like how the Horn works both as a replacement for random encounters and in the final confrontation. I think I would prefer if the Horn had different effects on the awakened hordes in other areas, for some variety.
Making the map the cover is always smart, and it gave you space to flesh out all locations sufficiently and it is easy to read all around.
On the first page, I would prefer if all the monsters were grouped together, perhaps doing Background+Barracks on left side, monsters and 1.Entrance right below it.
Curious about how you would decide which PC emits more heat?