I quite miss turn-based strategy games like XCom and Jagged Alliance, a genre that's pretty much disappeared from the world. Back2Back isn't one, and it doesn't quite satisfy the same jones, but at least in tenseness and overall aesthetic, it's along the same lines.
It's a level-based game; in each level, you control a handful of space marines in a fortified area at screen center, with waves of bug-like attackers moving slowly toward you from the cardinal directions.
Between levels, you can buy health packs and other consumables, although if you can you're better off saving points to upgrade your unit types permanently. During a level, units fire automatically at targets within range in the direction you're facing, so your only real action is to move them, generally from one side of the fort to another to face a different wave. Different unit types have different capabilities, so there's a combined-arms aspect to strategy.
Enemies drop things from time to time, and if all's clear you can dart out and grab them, but this is rarely useful because there's another wave coming somewhere.
In a sense, it's a tower-defense game, except that your towers are mobile; but it has a gritty, space-marine feel to it unlike most games of the genre. It also, in the initial cut-scene, has some of the most godawful voice acting I've ever heard, but never mind.
One thing I noted with interest was the use of virtual item upsell: You can, if you want, spend actual money for "premium upgrades" that improve your marines' characteristics very substantially. This is an interesting business model -- but perhaps not a smart one. That is, clearly I could spend real money to boost my marines to supermen and win handily -- but the accomplishment you feel in beating a game comes from overcoming obstacles by dint of hard work and smart play, and if you can simply buy your way to the top, the same sense of fiero is lacking.