Qincheng Prison, one of the world's toughest prisons, where notorious human criminals and other intelligent races were imprisoned. It is reputed to have a 0% escape rate over the past five decades.
The prison was located on the island of Qincheng, which is only about 6 sq. km of are. All it contained was a prison and outposts in every direction.
The prisoners here were completely cut off from the world. There are no visitations or releases. And they are known to have only two fates, either dying by trying to escape or turning into a bag of bones in the cell.
Because the inmates have a lot of reputation in the outside world, there would be always some fanatic followers trying to break into the prison to save their leader. This is why there are more guards around the prison than the number of prisoners.