Did a playthrough on Total Carnage of the single player levels.
Love the addition of the MADDs. They never did get enough "screen time" in the original game nor the mods.
Is the pistol not supposed to make sound when firing? It doesn't bother me, just wondering.
Elevator issues: throughout your levels you have elevators activated by players, but not enough ceiling space to jump down them. So, if the player wants to go down he has to jump on the elevator, wait for it to go up, and then wait for it to go down again in order to keep moving. You should definitely fix this, by extending the ceiling so the player can jump past it, or by having the elevator activated by a switch. Also, some of the elevators don't pause long enough at the top of their cycle; try giving them a longer delay so the player has more time to get off at the top floor.
In general I think your health, oxygen, and ammo placement could use a little work. Most levels need more ammo and could use more health.
Level 1: Brave New World
*Narrative level start. I don't generally like these. They're like an unnecessary prologue's to a novel. But it's got some nice lighting and architecture in spots.
*The first terminal says "You'll find new clothes on the table to your left". I turned to my left, expecting to see a shield canister, but there was nothing there.
*I don't know what the point is of letting the player access the lava area. Is there a secret in there? Because, I don't think it's a good idea to have an area that your player can accidently kill himself in during a narrative level that contains no combat.
*The area looking out at the Sphtkr looks nice, but some of the polygons need to be fixed. The thin pillars sometimes make the player "bounce" when he runs into them. Also, the BoBs will "clip" through the pillars when walking along this hallway; you can even see this in the screenshot of Brave New World that you posted to Simplici7y. Remember that monsters pathfind by drawing a line from one side of a polygon to another side of the polygon.
*I like the new BoB design. A bit more gritty and ready to fight.
Level 2: Sheep Among Wolves
*I remember the early versions of this you posted. A lot of improvements over all. Some new spaces that really add to the level.
*New Hunter bolts are faster and I like that you've made them red to emphasize how they're more dangerous now.
*I think you could add one more Hunter to the final room to add a little difficulty, especially since the MADDs are helping out.
*I think a shield recharger right at the last terminal would be nice, since the next level starts right in the middle of a big battle.
Level 3: Who Needs O2
*First battle is alright. I think you could remove some of the fighters and add some fighter snipers in the towers of the room instead. Would add some nice verticality to that battle, similar to another battle you have later on in this level.
*The battles in general are good. Most of them are big, open spaces with lots of enemies. It's classic Marathon fun, but perhaps they could use a little more variety in the combat space, maybe add some physical obstacles like benches or something? For example, that area with all the boxes and crates looks fun to fight in, but it never really gets used. I'd say move those crates to one of the other combat rooms.
*I think you need to add more O2 in this level. Either another recharger or more canisters.
*This level could use some extra ammo in spots.
*There's an area where it looks like the hull is breached to the vacuum outside, and then you can walk around outside. I'd block this off because it's too weird and immersion breaking to be walking around on vacuum outside the ship. Or at least that was my interpretation of it.
*There's a battle with fighters and blue fighter snipers and across a short bridge there's some Hunters. To the sides of the bridge are two pits that are suicide traps. I checked for secrets and didn't find any. You have to fix this. Either by making the floor of these pits teleport the player somewhere else, or by blocking off the pits so the player can't fall in.
Level 4: Veni Vidi
*This is your most ambitious level here, but I actually liked it the least.
*First, I think you may have too many monsters active at once and this is causing some to act weird sometimes. This is due to the newest version of Aleph One bringing back the active monster limit from Infinity.
*The start to this level is an interesting idea. You start in a set piece that is an infinite battle between Pfhor and humans. You infiltrate the facility but the battle keeps taking place outside; I like the atmosphere that creates. However, here are some problems. Right at the start I saw a Juggernaut, Troopers, and no real cover, so I immediately just ran away. Which means I missed the terminal, save terminal, and flechette. I eventually came back with yellow health and got the flechette and read the terminal, but still. Also, it's kind of cool reading about the battle as you can hear it taking place right next to you, but the player should be a little more safe at this start area if he's expected to actually stay and read (maybe hollow out the backs of some pillars to hold the terminals).
*This level needs more ammo and it needs a shield recharger instead of just canisters. A good rule of thumb: If there are infinite enemies then there should be infinite ammo and infinite health. I mean, even the Juggernaut respawns infinitely.
*The general outdoor architecture is very good. The whole courtyard bisected by the river looks great. And again, I like the idea of the battle set piece. The indoor architecture I felt ambivalent about. And the indoor monster encounters didn't really excite me. I preferred some of the monster encounters in previous levels. I think it's because in this level there wasn't enough space to move around most of the time (especially with the room with the four switches and the compilers).
This door has some glitches. When the player tries to run back inside through the right side of the door (circled) he will get "bounced" to the wall next to it (where the arrow is pointing) and will never be able to enter the door this way. The exact same is true for the left side of the door. You can only enter through the middle. This happens when polygons share a line (or when a line has too many segments).
Also, in the above picture you see the balcony above the arrow I drew. If the player steps down onto the lower ledge of that balcony he'll get stuck and can't move back inside. Probably won't happen often, but it's still an issue. I believe this also happens for the same reasons listed in the above paragraph.
These are some decent maps. You're clearly learning and exploring new ideas as you go and that's always good. I'm interested to see what new things you'll try in later levels.