You’ll initially fire up a squad or two and laser-zap viruses that wriggle like 2D worms on the surface of grid-covered 3D landscapes. While it’s easy to tell a squad where to go (left-click), or fire (right-click), figuring out where to hit the zig-zaggy viruses themselves feels a bit dodgy as you try to translate three-dimensional weapons fire onto a 2D plane. Arcade-tight Darwinia’s not, though when you receive better weapons like grenades, it’s easy enough to just hang back and lob explosives down inclines to nuke the enemy en masse.
Eventually you'll need to summon Engineers to grab control towers and collect the "souls" released when you snuff out viruses. Engineers drag souls ten-at-a-time back to Incubators where they're recycled into full-fledged Darwinians, but since you can't interact with them directly (hello Lemmings!), you have to convert a handful into Officers and position them like signposts to guide the remaining greenies to safety. Unfortunately, the path-finding AI is so silly that both your special units and Darwinians will often commit suicide, walking directly into force fields or viral orgies like robotic dimwits.