Very good improvement
I'll unfortunately have to review the game only for it's two first levels because at the last room of second level some bug hit on and the screen just turned white while aliens didn't even move (only way I could see this was because of screen turning to normal while pausing :D)
So the game was very much improvement from the original one and this is how the first one should've been also. I can only remember getting pissed at hordes of aliens spawning next to me at a dark area and having to run back to light to actually know that they're all dead in the first one. Now there's enought visibility but the dark areas are still used well. The maps are somewhat versatile with few different routes to go and guns actually killed aliens this time so you wouldn't have to run backways few rooms just to get the aliens killed.
Music was a really nice mood creator and a piece of info that there's still some bigger aliens alive and after you. The storyline was modest but worked out fine for a shooter like this.
What I didn't like so much was the predictability of the spawns as when you see a pile of ammo and guns in a room you can be sure you'll be attacked (tho good balance of predictability was is good for the mines to have use for. Whenever you push a button you'll propably be attacked on, when you go to middle of a new big room you can be sure you'll be attacked on and when you get new shiny and fancy weapons you can be sure that they'll go for good use in a moment so the big spawns were never a suprise as they actually kept coming all the time. At the same time the productive aliens were always introduced too late as the first one started to spit those little bastards on you in such a speed that you notice too late that it's vital to go for the spawner straight because you can't shoot the spawns down as fast as they come alive. On the first 'nests' I figured out there must be something creating the spawns there since they just keep coming so rushing to that corner revealed them and AFTER you kill them the game tells that you should kill them as fast as possible. What bothered me about this was that there was actually some alien containment chambers or such earlier so those or some computers or maybe some barely alive NPCs are used to tell that there has been spotted 'alien x' which do this and this and have a weakness in that so you have knowledge that there are going to be spawners coming and prepare by knowing it's there. There wasn't too many epic battles anyway since the number of powerful aliens spawning were quite the same in the suprise spawns and the 'arena' battles.
That's really a question about how do you want to keep the game. If it's going to be one where the point is to get guns and kill a pile of aliens then it's a successfully balanced game but it could be a game where the weapons are not so effective but there are less aliens coming on you all the time and when the big spawn comes then it's more stressful to use your weapons and surroundings in the most effective way possible to get out of that mess. That would also bring some of the difficulty some people have been whining abouthere but I don't know if it would slow the game down too much to be an exciting one.
All in all you have made a really nice package to keep players entertained and a bit on the edge.
What I was left hoping for was that there could be some more interactive solutions like having tasks on the map so you could pick an alternative route to exit or i.e. optional task of operating some fuel injection/burning system which would then open up and ease the final room by scorching some alien nests or so.