Azkaban

Azkaban is a prison in the Wizarding World, managed by the Ministry of Magic and guarded by Dementors. This is were the foulest criminals are sent; one of its many prisoners was Sirius Black. He was later redeemed and set free, as Severus Snape, following the orders of Freedom, told the Ministry of Magic that Sirius had not been the one to betray James and Lily Potter, thus causing their death. That man had instead been Peter Pettigrew, who was found in his Animagus-form near the Burrow and sent to Azkaban in Sirius' stead.

Azkaban was unguarded for a while when the dementors left the place to serve Sauron in the Magic Realm, but the Ministry of Magic soon found other guards, but its not mentioned what kind of creatures these new ones were.