Alien Hallway is very aptly named. It takes place entirely in hallways and it has aliens in it.
The game is a simplistic tug-of-war. And when I say simplistic, I don't mean it's been dumbed down, I mean whack-a-mole has more complexities than this. You spawn soldiers and they automatically walk down the hallway towards the endless alien spawner. The only decision you have is when your soldiers should use their grenades but even those are on a cooldown so it's no big thing if you waste a few.
In between missions you get to spend the money you earn on upgrades in a small metagame and as the game progresses more soldiers and aliens unlock. The gameplay however, never changes.
Because everything is on a tiered cooldown so you will never have any choices to make beyond which soldier to build first. Everything after that is simply bought when the cooldown ends, not when you need or want it.
Alien Hallway is a perversion of the genre and so easy even a child could play it and win.