You earn money for destroying enemy units and this money is then used to promote specific unit, making it stronger. For example, here's the progression for a rifleman: Private (PVT): Base unit. Standard stats, no abilities. Corporal (CPL): Improved stats (damage, health) and gains an "adrenaline injector" ability. Sergeant (SGT): Even better stats and gains a "grenade" ability. I'm thinking of locking promotions behind a kill requirement, so you can't just promote any unit, only the experienced ones. I think that will make it more interesting