Siege Foundry is having another go at it, and it's heading to iOS soon. You pick a chassis, fit wheels, and stack up to five armoured floors on top, each one armed separately. Each one carries its own health pool, so when a lower floor goes, everything above it drops into the gap. Which means your neat five-tier design is a three-tier one with the wrong weapons at the wrong heights, halfway through a fight you were winning. Upgrades change how weapons look as well as what they do, so a machine that's had a lot poured into it looks like it has.