Infested Planet is a neat little tower defense/action game where you control a bunch of soldiers and you need to complete objectives that most often than not involve destroying alien bases and taking them over while hordes of enemies move toward you. I had a good time with it, even if I'm not a fan of the upgrade system/general and that gameplay often felt confusing.
Your squad of marines can't do much at first. You move + attack and they'll walk towards a point, shooting anything that comes into range. You start from a base - and that's where your marines will respawn if they die - and most often than not, you have to destroy alien hives and stand on the unoccupied spot in order to claim them as yours. The aliens have tons of stuff they can turn against you; Long range turrets that poison your troops, random mutators that boost them in decisive ways and sheer numbers.
You get upgrades as well; You can equip your marines with different weapons - shotgun, sniper rifle, flamethrower. You can build structures that allow you to call air strikes, recover health or ammo more quickly, you can also build very important turrets to keep spots of the map protected even if your team isn't there. You unlock that by using money you get for completing missions, but then you need to individually buy each upgrade with 'Command Points' in each map, and that's true with most upgrades. That's why I was wary of buying anything since you can't buy everything at first and most of these are pretty situation-based. There is one upgrade that turns all of your bunkers into healing beacons, but no other upgrades are permanent and 'passive' like that.
Worst yet, you can buy single-mission upgrades that give you weapons or medic pods. I wish there were more passive/permanent upgrades you'd get to buy for your troops instead of simply 'unlocking' them. You'll probably need some of them to progress through the game since air strikes are very powerful against buildings and your rocket launchers put your units at risk. I like the ammo system IP uses, you have limited ammo to use certain abilities and it recovers over time. I don't like how your marines will just target everything they can when using the default rocket launcher, however, makes it hard to blow up turrets and nothing else.
In general, IP has some weirdness on the control side. You can select all your units and have them move/attack around, but it's tough to do precise things, such as targeting one structure in particular. Alien turrets can mow down through your squad at very long ranges - longer if they get the mutator for that - and you have to use rockets or helicopter strikes to get rid of them quickly - and they regrow after a while. That being said, the game is pretty forgiving since your units can respawn continuously. The command point system is also interesting since you get them from capturing alien bases - or finding boxes with them in it - and it's used to buy pretty much all your upgrades. You can recycle everything at 100% cost, so you can try whatever you want, move your turrets to a better spot, etc.
Infested Planet is pretty good! There is some depth in it and some satisfaction of mowing down waves of aliens to get to their base. It's like Starcraft if you were only Marine against Zergs, pretty much!