Commentary on Marathon 1 is by no means new; I will try to link to the Tour of Duty for the levels as well. I will attempt to focus on improvements that could be made since that was not in the mind of the Tour of Duty authors when they wrote their commentary. There still will be commentary of my own, though. Despite the naming similarity to Ryoko's commentary, I'm probably not going to go in that direction. Since it is the original Marathon you won't find much architecture that gets good scores out of 5. Besides, I don't want to be mad at my favorite scenario of the trilogy.
The difficulty level I am playing on? There's only one true difficulty level.
If I refer to you, I refer to anyone who makes the maps for M1 Redux though that may be one person in particular.
LEVEL 0: ARRIVAL
Tour of Duty Link: http://forums.bungie.org/story/?read=62049
When you first enter the level, you've got a tough fight (unless you are playing on an untrue difficulty level, which is okay if you are a noob). I found that I was out of shape when it comes to fisting fighters; you can easily die here (well, I did) even if you are somewhat experienced with the Marathon franchise. I like the difficulty. If nothing else, it's a good way to discourage newbies from putting the difficulty on TC. It's just a good note to start things out on regardless.
The first terminal you read is a standard emergency broadcast. "All personnel are required to arm themselves and fight for their lives."
While I am discussing the very beginning, I should mention the idea of having the player start in the hangar. How would you make the shuttle? It might look a lot more obviously 2D if the player got all up close and personal. The player would then presumably come in through the door to the hangar and not from where the player currently starts. It would alter gameplay some, but it might not be a terrible idea. On the other hand, if I recall the manual made clear that Durandal wasn't being very cooperative, and maybe the door that you would normally use just isn't functional. The problem is I'm not sure how you would do the presumed space walk that is required to get to where the player currently starts. Just something to think about.
I'm sure someone has said this before, but the architecture on this level is for the most part really good. Seeing into other parts of the ship is just good design, and really helps it feel like a real place.
Atmospherically, the music of the level combined with the stark contrast of light areas (say, coming through the window) with dark ones really sets the mood of the entire game out on the right note. Combined with the cramped conditions of the maze, you really get the feel very fast this isn't one of those games where you are a hero with epic music while you do massive deeds. Rather, it is a somber tale where you don't know what to expect around the next corner. That being said, you might be able to change the music here without ruining things too much, and if you use a lot of Craig Hardgrove's music it might be better to start things out with one of his pieces.
It's important to keep the space in the rooms with the windows, since you need it to dodge around the fighters. Ideally, of course, there might be some chairs or other obstructions since I imagine the first rooms as maybe common rooms or welcome area type of places.
It's worth noting it's very easy to not have to hit that one switch to get into the maze area since you can just "jump" around the pillar, though I'm sure everybody does that.
The maze itself is presumably some kind of maintenance area. The terminals are titled "Engineering access," so you could add say some sewage canals (maybe not large enough to go into though) to make it more clear this is a utility area. I also think there is a certain fan texture that would serve the area very well. If you wanted to spookify the area a bit, you could add like a single area with flickering lights, though do not go overboard with this. It could really set things off on the right note.
I'm not sure what the function of the area is where you jump out of the maze into. It would be hard to change this though without interfering with the fighters or the secret that's here, since you can't really modify the walls because of the secret and you can't really do too much to the floor lest you get in the way of the fighters. Maybe the whole area needs to be modified and redone, especially since it leads into what should be the hangar control room.
Other than standard things you would probably do without me saying (adding extra nonfunctional doors to make it feel like the ship is larger than it is, adding security camera images to the terminal maps and other nice pictures, etc) I'm not sure there's much more to say. After you run through the maze a few times to make sure you got the secret and are full on health (I'm not the best at this game, okay?), you go on to BGN. Maybe make things a little more convenient so you don't have to make so many circles through the maze (say, a health charger in the hangar control room), or am I just bad? Probably the latter. If I was better I could have avoided needing the health charger the second time in the first place.
Arrival is a pretty good level overall in my opinion.