From the description in the Store:
Intergalactic Council has assigned you a task to defend an important space station against endless space pirate attacks. You will command 4 space officers. Battle Officer is in charge of Rocket Turret and Scanner. Tactical Officer manages Shields and Shield Recharge, and also she has tactical features like pumping energy cells into the space station's battery. Plasma Core Engineer works with Plasma Turret, improving its damage, aim and energy consumption. Laser Weapons Engineer is in charge of Laser Turret, which may be weak in the beginning, but once the engineer learns her skills, it will become on a par with other space station's weapons.
Space pirates may be weak in the beginning, but with time they will strike you with all their might. Their space fleet is large and they possess 4 types of frigates, 5 types of destroyers and 2 types of battleships. Pirates have missiles, torpedoes, lasers, plasma beams, invisible destroyers, and support and repair ships ready to destroy your space station.
Sounds complicated. And it is, ultimately, though you can get quite a long way with just random upgrades and tapping in the right place to aim your station's weapons:
I was surprised not to see a 'freemium' method of buying extra credits, but credit to the developer for keeping this free and balancing the earning of the aforementioned credits with the need to keep improving abilities, so as to match the ferocity of the pirates. So yes, a numbers game at heart, but dressed up very nicely in space livery.