Unreal City was a massive, cube-shaped Techno Union prison on Metalorn. Large enough to hold twice the population of some planets, its appearance alone was enough to prevent rebellion. Wat Tambor also mentioned "The mere sight of it has workers toiling twice as hard. You can actually see it from orbit, so even our space stations are built more quickly," during a tour he gave Bail Organa.
Shaak Ti was once detained in the high security area of the prison near the end of the Clone Wars and with help from Bail Organa, became the first person ever to escape the high security area. The cost of housing an inmate in the high security area was said by Wat Tambor to be astronomical.