You sit in the middle. The gun ring turns on its own. Keep the fence standing and the dead off your lap.
Tap or swipe over glowing energy to collect it. Enough energy freezes the game so you can pick a weapon.
Tap the bunker to use Overdrive once it is charged, or set it to Auto and it fires itself. It sends a shockwave, spins the ring faster and doubles fire rate.
Tap a cracked or broken fence panel to patch it.
Fire mode
Overdrive
Keyboard: Space is Overdrive, F switches fire mode, O switches Overdrive mode, P pauses, 1 2 3 pick upgrades.