Alright purist here's a two-pronged post dealing with both your level and mine.
As always, here's a first-time playthrough demo of your level on UV. If a picture says a thousand words, then a demo must be the complete works of Shakespeare. Watch out, it's a long one (45 minutes-ish I think?).
In case you want to watch the demo first, my comments are in spoilers.
Now onto MAP25.
I've played both versions of Sacrifice. It's a really nice level. In terms of difficulty it has spikes and troughs, which will catch players off guard. Thematically, it fits in great - though I'm not sure what the title sacrifice refers to? One of my concerns was that HAD24 has lots of cells and the runover may spoil your map. I don't think it will be a massive problem having played your map though.
The title refers to that final room with the sacrifice on the altar where the red key is, presumably a sacrifice to the Icon of Sin? His picture is on the walls surrounded by candles. That's what I was going for anyway, but I sort of hashed it out as an attempt to make my level at least somewhat relevant to the story. If anyone has better ideas, I'm all ears. I should actually write that story at some point, but I'm not really sold on the idea myself. I guess it would be funny if the Icon was like a god to the demons, who knew they were religious?
Here's my notes:
25.3 Not sure on the spectres in the blood cavern. They don't really cause any bother once you know they're there (unless you miscount) but the first time you play they are a bit of an annoyance. Not a dealbreaker by any means.
I like the spectres because sometimes they find their way out of the cavern into the main area and sneak attack you, which was an accident but pretty awesome I think. However, not being able to see them half the time kind of blows so I've met you half way and turned them into regular demons. I've also changed the bloodfall in the "ledge" area so that if you want to go down there from the ledge you can actually shoot them instead of have them block you off. Makes it a bit friendlier for the player.
25.5 In the treble AV area (survivor of three revisions!) a degree of light variation or overall darkness would add a much needed forboding to the atmosphere.
25.6 In this same area I was finding it much tougher than I did before for some reason - has it been changed at all? Smaller pillars or maybe? Did MAP30 have a BFG to fight them with?
Agreed with the lighting issue.
For the difficulty of the fight, MAP30 always had a BFG available for it which really changed how it played, but with the rest of the level how it is, I don't really want to give the player a BFG or it'll unbalance the rest. The pillars are the same width as they've always been. Given that a lot of players will be coming into the level with a BFG and a bunch of cells I think it might be OK to leave as is, but it's very useful to know that other people find the fight to be super hard. I'm not really sure what to do about it. :(
25.7 I like the rev raising from the blood but you might want to change the texture on the raising floor as it doesn't match the surrounding blood.
25.8 I'd consider changing the chaingunners that ambush the blue key since they attack you from both sides and there's no cover.
The raising floor already has a blood texture :p There was a little grate that the blood drained away into but it won't hurt to remove it. I've also swapped the chaingunners for imps.
25.9 The small room where you get the yellow skull key needs a more hellish texture surrounding it than techbase grey. In this area, as well as the co-op issue, could it also break if a single player was stupid enough to leave the room and trigger the lava without picking up the key?
Wow well spotted. This is an easy fix but yes it was possible. Also the walls are red, not grey :p but I'll swap them for something more hellish.
25.10 The lava cavern could do with some health. Maybe a bezerk to get the player back up to 100% since it hard to keep ahead of the lava and it's so damaging. I'd also replace or remove the arachnatrons. At least reduce it to one. There's no cover so they liquidate me every time. I assume you can do it but I couldn't so I god moded past this bit. I still love the concept though.
Instead of adding more health I've decided to try out making the floor less damaging. I've also removed the three spiders and swapped them with two more mancubi but, I've placed them so that you're only really engaging with one at a time. Far less of a deathtrap now.
purist said:25.11 In this same area I just dash up the rocks as best I can in a panic until I reach the teleporter but I noticed some gargoyle head switches. I see in Doombuilder that they are switches but I can't figure out what they do?
25.11 You need to add some secrets, or at least one.
The gargoyle switches are actually sort of a secret, they give you a BFG if you hit both, but I'm gonna change that to 200 cells because I don't really want the player to have a BFG. Originally I was planning on having a voodoo doll move over a secret floor if you do that, but of course it doesn't award you the secret so that doesn't work (I assume the voodoo doll gets the credit?). I'll look into adding another secret into the level (maybe a berserk pack).
25.12 I agree you need new music as the stock music doesn't go either. PRIMEVAL has been a useful contributer in this regard. I could ask if he's free.
I'm always up for unique music but failing that I think this song might be cool (no hotlinking, copy it into your address bar): vgmusic.com/music/console/nintendo/gamecube/Eternal_Darkness_Final_Boss.mid
I'll upload a version 3 of the level when it's done. Got some ceilings to pretty up :)