The Prison was an Unnamed Location northwest of Moisty Mire (and eventually Paradise Palms) and east of Fatal Fields. As the name suggests, it was a prison complex with four watchtowers, each of which could spawn a chest. It had up to 8 chest spawns and decent loot, and it was usually mildly contested. The most prominent Material found in the prison was Stone.
On May 1st, 2018, with the release of the v4.0 update, the center of the complex was mostly demolished by a fragment of the Meteor. One of the guard towers was also leveled, with only the first story remaining intact.
In Season 10, the location was brought back with Moisty Mire, now named Moisty Palms. The only real difference between the old prison and the new one was that the prison now was located in the desert biome, instead of Moisty Mire.