You're building the engine while the zombies close in. Place gears in the grid, chain them back to the starter cog, drive faster. When the grid fills up, that's your problem.

Machines run when you're not there. Come back, merge two into one stronger one, collect what built up. The factory runs itself. You just make it better.

Your units fight on their own. Drag two matching ones together and they upgrade. That's it. Wave 7 will make you feel stupid.