From what he can see of the orbs, they appear to be large, floating, tethered, organic spheres, fillwed with something smaller, that floats through it in aimless patterns, issuing for the light, which is diffused by the outer bubble.<br/><br/>His examination of the city gets him a good idea of the layout, and he can see the location of the entrance to the vault, near the roots of the second tree, the one that's not hollowed out. It looks as if the vault extends below the tree, into the root system.