From "WikiDex: The Free Encyclopædia"
Retrieved September 19, 2013
Azkaban Prison: Security
In addition to its remote location in the North Sea and the wards (permanent magical effects that prevented teleportation into or out of the prison as well as most forms of magical flight), Azkaban was guarded by Dementors, like many other magical prisons in the Western World. The Dementors provided security in two ways, by suppressing the emotions of the prisoners, greatly reducing the chances of an escape instigated from within, as well as providing a roving patrol unimpeded by the anti-flight wards.
From the time of its establishment in the 15th century until the integration of the British Ministry of Magic into HM Government, Azkaban was famous for being "escape-proof". The closest to an escape happened in May 1983, when Sirius Black, having been wrongfully accused of treason and fourteen counts of murder, though not having had a trail,, "released himself under his own recognizance" with the help of James Shepard. While technically an escape, the British Ministry of Magic officially maintains that since Black wasn't convicted of any crime, it was not an escape. That nearly half of the prison's inmates at the time were killed, all convicted Death Eaters, allegedly by Shepard and/or Black, is officially claimed a coincidence by the British Ministry of Magic.