Alien Armageddon Zone Act 1:
My first reaction upon loading this level: "Wait, I thought that was castle..." Really, this zone changes themes every half minute, and it's not doing it any good. Stick to a castle theme, and if necessary, make this zone come up earlier in he rotation. Either way, the texturing in the beginning is horribly monotonous and bland. Also, what are Crawlas doing there. I guess I get your point, since this is Eggman's core residence, you probably want all enemies to show up. But it just doesn't work. Stick to castle enemies and you're fine.
The next room shows another problem that actually was apparent as early as Fume Shaft. Two different liquids in one level is a bad idea, and it hurts consistency. The rock section itself is OK though, it's nice and challenging, even though the room is ugly,
Give me one good reason to have flamethrowers and rock spawners in a castle level. Really, put these into Fume Shaft where they belong and concentrate on Maces and Chains. I bet you can get more creative with them as you're currently doing. The cannonball section also couldn't be more pointless, the cannonballs don't actually fall down again. The key to making a good cannonball section is forcing the player to stop dead on several occasions, so that the cannonballs actually become a threat.
The next cave-like section is largely uninteresting due to lack of visuals and enemy and flamethrower overplacement. The next real offense comes with the Detons though. Instead of coming from behind, they are in front of you, crashing in your face without any chance to escape. It doesn't help either that the room is cramped and full of rocks.
The underwater section needs to go, no discussion. It's overlong, empty, bland, cramped, unfitting, too dark and simply boring. In short, it has no redeeming value. I do like the boat room after that though, in terms of visuals. You can do good visuals if you try, but don't try often enough. The extra lifes in there must be a joke though. The pseudo-river basically screams "WATERSLIDE!" to me, and I actually think that waterslides combined with maces are very cool. I don't like the blandness and crampedness of the path split after that though.
The texturing in the quicksand has to be another joke. Seriously, one single textures for everything? Hell no. I like the quicksand idea, it somehow looks a bit like blood. It's not used very creatively, however, and the theme needs a proper transition to the castle. It just feels randomly thrown in there. The enemy ambush room is pointless and boring, and the chain after that goes to the floor which makes it hard to grab. The fireballs in the next section are too easy to dodge, and the enemy choice is catastrophic as ever. Actually, coming to think of it, the volcano-mansion transition somehow worked in KOTE's Eggmansion, so you might wanna take a closer look at how he does it. You also shouldn't use lava, it doesn't fit the theme. Stick to your quicksand instead.
The swinging chain in the next room can be ignored, and the lava makes the room look even worse than it already is. But then we finally get to the real castle. Ignoring the fact that the liquid changes again for no reason, this room looks quite good. Not a work of genius, but OK. I like how you walk on top of the things that emit sime, and that could probably be expanded upon. I don't get what the mines are doing in the air though, and after that, we return to green slime and bland rocks without proper transition. Actually, that whole section is totally pointless and boring, but I like the door swing at the end, even if the room is much too bland.
Alien Armageddon Zone Act 2:
The visuals at the beginning are a nice try, but the wall texture is too repetitive. Also, the maces have been doing the exact same thing since they were introduced, and it's time for something new. They should be used creatively. The same rules as in the last act apply to the cannonball launcher, they need to involve the player. The light room after that is a cool idea, but as Mystic said, it's too long and the blue floor looks bad. Maybe flood the insta-kill sector with a red colormap or something. The mace section after that is pointless, and I hope it's filler.
The 2D section does some very cool things, and apart from some missing texture variation, it has some neat visuals as well. You might wanna remove some enemies though that can't be seen before you jump into them. Whatever the PolyObjects after that was supposed to do, it doesn't do it and instead glitches up. The library part looks quite nice, but the maze was really not necessary, especially since there are arrows anyway. In general, the underwater section feels boring, and the transition at the end doesn't work at all, especially since we return to a castle theme anyway after one room.
The next section has even more (already boring) maces and a weird elevator room that doesn't do anything. The scrolling floors after that are largely unnoticeable because it can run at full speed anyway. Also, they look weird, and the Mines are too easy to avoid.
The next section is solid, not too interesting, but not harmful. There are way too many staircases, however. Also, the Robo-Hoods on the pillars look plain weird, and the platforms in this room can be skipped easily. Then we have more useless cannonballs launcher and weird Mines in the air for no reason. The transition to the quicksand works much better now, by the way. The next few scrollers are pointless because there is nothing that could possibly harm you, and the end is too anticlimatic.
I can remember the staircases being flooded on my Co-Op playthrough, but not on my SP playthrough. The water probably goes away when I die and respawn, but it was horrible anyway.
Overall:
This beast can't decide what it wants to be, and it gets boring and bland way too often, regarding both visuals and gameplay. It has some neat ideas, but it's not very enjoyable overall.