Aging robot defenders jam, overheat and rust; oil-can maintenance clicks, parts-swaps and conveyor-fed ammo keep the line alive, and scrap value is your economy. Signature: cannibalize a dead mech to revive three others for the last wave.
Rust oranges and chain-link greys, factory spark showers, oil stains, warning-stripe accents.
Feral junk-machines march the floor line toward your home reactor. Your defenders are old mechs: they jam, overheat and rust until they seize solid. Keep the oil-can swinging, parts-swap upgrades, and build beside the conveyor — it feeds their ammo. On the last wave, cannibalize one dead mech: its guts revive the other three.