Fast paced RTS/Tower Defense hybrid. With the player piloting a transformer mech.
Most of the stuff can be earned, and what there is to buy with real money is cosmetic, mostly. While the game absolutely does want your money, it doesn't get ugly about it.
Gameplay consists of you rushing around the map doing things such as collecting units (heavy units can be given Capture or Kill orders or just dropped to guard), deploying them in position, or battling enemy forces in person. You level up your mech during battle which allows you to build upgrades for youself. In addition you can build and place turrets or artillery pieces, drop mines, bombs or even bear traps. That's before we get into the funky stuff like stealth, healing or straight up stealing enemy units.
Game modes are Solo, Co-op or PvP in 1v1, 2v2, 3v3 or Survival modes. While there is an initial learning curve, AirMech is not an extremly complex game. I'm more of a fan of turn based combat but the quick pace of AirMech suits me just fine. Your mileage may wary. You earn one type of currency, the other can be bought with real $$. You spend currency unlocking units to build, mech equipment or buying cosmetics or pets and such. You also get random drops after some battles.
What are you waiting for? Transformers (not in name/license obviously), free, good! Try it, you may not find the best game ever, but it is average at worst.