Alien Hallway is a real time strategy game played out like a tug of war between you and the aliens. If you've played Swords and Soldiers, you get the idea.
You gather resources with engineers to produce more and more, we'll call them Space Marines of various types to push back the alien onslaught and eventually destroy their teleporter and prevent the aliens doing the same to you. Units come in various types from the cheapest Flame-throwers who stand toe to toe with the aliens to the more expensive longe range, fragile but high damage units like Snipers and Missile Launchers. The basic jist of the games is to produce lines of various units to not only do damage but to protect the high value, high production time units at the back. Flame-throwers and riflemen are cheap don't do a lot of damage, but die well to protect the better units.
Completing a hallway earns you gold that can be used to upgrade you units and improve various other aspects like earning more gold when you complete a level, decrease the time it takes to produce units and have the engineers collect more energy. The mechanic is similar to Defender's Quest where at higher difficulty levels you will need to grind levels multiple times to be strong enough to move on. At normal you can just do one level after another without too much trouble.
The graphics are serviceable at best and the game could use some usability improvements. You can't use key-binds to produce units and the default mouse pointer is mostly black against a black background and can be hard to see. Also don't try to take a screenshot, it crashes the game to desktop.
All in all, if you can get the game for cheap, it's decent fun for a few hours. Don't think about paying full price.