That wheel thing is actually the cereleum engine that powers them - it seems to work like an aeolipile, an ancient Greek forerunner to the steam turbine, or else like a catherine wheel (that's why you see aetheric jets of gas shooting out of the 'prongs' on it, it's what's making it turn and thus provides the propulsion).

Anyway, getting back on track, Catapult and Wyssahtyn are totally correct - it's wreckage from the Garleans' first disasterous attempts at storming the Black Shroud before the Calamity, and of course, the forest keeps it's secrets.