There are cracks under it on the City side, so we can see into it perhaps. Is the Traveler hollow or is it more solid than not? On the solid side, it might be something like a a small moon - (That's no moon). On the hollow side it might be a habitat with ringworlds in it. Or, maybe it's a shell world. Or maybe it's a portal of some sort. Or maybe it has a dwarf neutron star in it.