The city is a planet-wide city a la Coruscant. Problem is, the planet-city orbits TON 618:
As a quasar, TON 618 is believed to be an accretion disc of intensely hot gas swirling around a giant black hole in the centre of a galaxy (...) The surrounding galaxy is not visible from Earth, because the quasar itself outshines it. With an absolute magnitude of −30.7, it shines with a luminosity of 4×1040 watts, or as brilliantly as 140 trillion Suns, making it one of the brightest objects in the Universe.
The only reason the city does not burn to a crisp is due to a post-singularity shield around the planet. Leave the planet, though, and the radiation from TON 618 will disassemble any gold you take with you into subatomic parts. You will be disassembled too, by the way.
Cort Ammon had a very interesting comment about TON 618 last time invoked it:
I was going to write an answer about how you could use a supernova to generate more power. They output about 1 foe (1044J1044J) over 20ish days. Over 20 days, TON 618 outputs 200 foe! That's insane! That quasar literally outshines supernovae! – Cort Ammon