"Get ready to send marines to kill aliens that no man's seen before!"
That, my friend, is the plot of Alien Hallway in one sentence. Sorry for spoiling it. Alien Hallway is a light strategy game where you only manage the finances and hiring troops, who handle the destruction by themselves.
Drones get you money by scavenging electronics. With money you can send marines to destroy the alien teleporter, the objective of every level. The marines are AI controlled and you can only choose, when to throw their grenades. At the same time you must not allow the aliens to destroy your base.
Apparently, these aliens are fairly technologically advanced. There's no telling which side threw the first stone but because of the mentality of the storyline, my bet would be on the humans. Most of the aliens are fairly mindless and naked green little men who just come and smash your hyper-trained marines but their elite troops have powerful swords and advanced armour, ranged weapons like blasters, forcefields that make the aliens nearly indestructible and even huge exoskeletons with tank cannons attached to be reckoned with. One alien type clearly tries to fry the brains of your troops with some kind of telepathy and another one is an exploding kamikaze troop.
On your side, you've got a fairly diverse bunch of soldiers and a manually used air strike that can be used every once in a while. Curiously, the flame-thrower guy is the cheapest and weakest of the bunch. The pricier ones are generally better, so later I ended up usually simply stacking up on snipers, missile troopers and heavy laser troops to beat the levels quickly and efficiently. This is important because it gives you gold.
In between missions you can upgrade your troops and other assets with the gold, which in itself is pretty cool. Higher upgrades are more expensive with diminishing returns on investment. The most significant upgrade is the soldiers getting better grenades at level 5, otherwise they just do more damage and can take more beating before dying.
All levels are really similar to each other, with just stronger varieties or larger amounts of mobs. There are a mere couple of special ones like the one where you start with a large amount of funds but no income. There's some replay value to the levels because you can do them in four different difficulty levels - indeed, one of the achievements for the game requires you to beat every level on the highest difficulty (Expert) level. You can also try to 5-star each level, which amounts to doing them fast enough or with just a small number of casualties. Unfortunately, it's not very obvious what's enough for 5 stars, but the above goals seemed to nail it for me.
Too bad the game is very grindy. To survive the last levels even on easy requires a good bit of grinding to get powerful enough troops to survive the missions. You can redo missions for gold, but it's rather boring because you already beat them before and they're no different when repeated. To get just one troop to level 10 requires on hour or more of grinding and to survive the expert mode you need several more troop types to high enough level to survive. One could call it a challenge, but mostly, it's a grind.
This might not be a problem if you could control your soldiers a bit more. Generally they shoot the closest enemy and walk towards it, and the same goes for the aliens. However, often times it would be very useful for the snipers to concentrate fire on the forcefield units or the big enemies, but they will also just target the front-line. This is especially frustrating, when the alien teleporter is very close to being destroyed. The troops keep shooting some big tank enemy that could be finished off after the teleporter was down, but before the creature is down more enemies pop out of the teleporter. Just general orders to "target large enemies" and/or "target the alien teleporter at all costs" for different troop types could have made it so much more interesting.
My stance to not recommend Alien Hallway stems from the lack of control over troops, because it's not enticing to just buy all the snipers, missiles and laser dudes your money can get, if you can't give them useful commands (throwing a grenade is not really a good one). This lack of control also makes the game rather casual. I got some 11 hours out of it, but probably first 2 or 3 of them went to finishing the game, the rest being just grinding for gold. It's not a bad game, but it's not very interesting one either. While its base price 5 € is not too bad, I'd still advice to get it on sale.