At first glance we thought Attack of the Earthlings was like a reverse Xcom, where you play the aliens instead of the humans, fighting for freedom in the face of a technically superior invasion force, and while it is exactly that in many respects and has a lot in common with the Firaxis series, the two are actually quite different from one another.
While Xcom has a deep and involving strategy layer where humans fight aliens on Earth, in AotE the shoe is on the other foot and the player takes control an alien happily minding its own business when along comes a giant drill-shaped spaceship full of corporate British dickheads. As for strategy, there's none of that here beyond a short four-way skill tree to unlock a handful of new abilities as the game progresses.
In each of the game's missions, you start with a matriarch who can spawn other aliens. The creature you initially spawn is always the same, unremarkable grunt, but if you've got enough biomass, the resource needed to make upgrades, then you can mutate that grunt into a more powerful creature. Then, with your alien queen and her minions in tow, you advance through various workplace environments, taking out guards and anyone else you encounter for that matter.
As far as gameplay goes, while turn-based and borrowing many of the mechanics found in Xcom, AotE is much more linear and authored than all but a handful of the more narrative-driven Xcom 2 missions. The levels are custom built, the guard and civilian locations pre-defined, and there's not a huge amount of choice when it comes to how you navigate each level. There is tactical action, however, and this comes from how you advance through the grid-based environments, although not so much which direction you take.
While it pays to be subtle, this isn't a stealth game in the sense that your ultimate goal is to move through a level as if you were never there, rather you've got to remain undetected until the moment you strike and kill your enemies with brutal efficiency. The thing is, you need to kill your opponents in Attack of the Earthlings; death is the fuel that keeps you moving forward.
Each corpse, whether it be alien or human, is food to be consumed that can, in turn, be used to spawn new units and then upgrade them, creating a constant cycle where you turn fleshy humans into alien beasties, as well as recycling your own fallen units as and when you can. The queen is at the heart of things, and if she dies it's game over. You can lose your minions though, but obviously it's better if you don't, because if you can build up a small squad then you'll have more options.