The Experiment is the second expansion for Alien Shooter, and it happens to address some issues of the original package, as well as some of the complaints with the first expansion.
However, the downsides of this expansion should be mentioned first. Firstly, there is no new weapon to be had. Considering that this game is plenty about shooting aliens, that there is no new gun to be had would be quite a disappointment. There are also no new power-ups. The missions also remain the same: wipe out all aliens in a level, or blow up teleporters that bring in alien reinforcements.
The expansion also does absolutely nothing about the problems in the pathfinding and shooting A.I. scripts. The couple of new aliens – more on these shortly - only serve to remind Alien Shooter veterans about this. However, these are just about the only major complaints. The Experiment has more levels than Fight For Life, and more details in these than either Fight for Life or the original. This is apparent right from the first level, which is again set on ground level, but apparently have better looking buildings than those seen in the first levels of Fight For Life and the original. Unfortunately, these buildings, which are large, do nothing for the issues with the camera and view obstruction. In fact, they obstruct the view even more than the bland-looking buildings in the previous Alien Shooter entries.
There are also new objects like streetlights, bushes and benches, which are a nice visual touch, but like the new objects in Fight For Life, these have odd hitboxes that can make going around them and shooting through them problematic, or at best unpredictable. For example, the aforementioned benches have thin hitboxes that prevent aliens and the player character from moving through them, yet shots can be fired over them to hit any target behind them. Another example is a press bench in a gym-like level, which should be similar in stature to a sitting bench, but apparently it cannot be moved through or shot over.
Nevertheless, every level appears quite different from the last one, and the next one, which provide some visual treats.
Although there are no new weapons, there are new enemies to be had, which in turn made some once not-useful weapons actually useful in The Experiment. One of these new enemies is a large bipedal creature with a huge head and maw to match and spindly arms; this is a very tough and fast creature, but is large and unarmed. The player would soon learn that the Flamethrower is the most efficient gun against this brute; previously, the Flamethrower was an especially situational weapon.
However, when it first appears in The Experiment, it would give the player a reminder of how sloppy the pathfinding A.I. for the aliens is. There is no Flamethrower in the same level, and the player won't have much in the way of heavy weapons to deal with it. Therefore, the player will likely have to resort to cheesy tactics to defeat it, which invariably involves getting it caught in level geometries while plinking away at it with small-arms. To do otherwise is to simply get trapped, or attracting the attention of other creatures that are certain to add greatly to the complication.
The other new creature is a bat-like alien that appears to fly and can spit acid. In-game, their flight capability won't help them go over obstacles; they still have to go around it. However, they have hitboxes that are rather different from those of the other aliens; they are quite difficult to hit. The player would learn quickly that the shotgun, which was not very useful in the latter levels of the original game, would be quite useful against these creatures, which incidentally become more numerous as the levels go by.
However, the giant alien bats also remind the Alien Shooter veteran of the other perennial problem with alien A.I. The bats' acid attacks have slight splash radii, so the player can dupe them into shooting at walls and killing themselves.
In The Experiment, there appears to be more appearances of the autocannon turret, which was rather rare in the original game and Fight For Life. The turret was a very powerful weapon emplacement that trades the player character's mobility for greatly increased durability and tremendous, unlimited firepower, yet only appeared in very few places in the previous Alien Shooter titles, usually when there is a deluge of enemies to be fought off. In The Experiment, they appear more prominently.
Lastly, there is an actual boss fight to complete this next instalment in the story (which is again about a human establishment that has been overrun, no thanks to the namesake experiment). To describe this boss fight would be to include a spoiler, but it has to be mentioned here that it occurs in an uncomfortably small arena, which conveniently has a lot of ammo for the MAGMA Cannon. It can be a bit tedious, but then there never was a boss fight in the previous Alien Shooter titles.
In conclusion, The Experiment doesn't do much in offering anything new to the Alien Shooter franchise, but there is some obvious improvement over the previous games in the form of more detailed and better designed levels, as well as a couple of new enemies.