Azkaban Fortress

"They don't need walls and water to keep the prisoners in, not when they're trapped inside their own heads, incapable of a single cheerful thought. Most go mad within weeks."
    --  Remus Lupin, PA10

Introduction

Far out in the icy waters of the North Sea lies a tiny island that is home to one of the most terrible places on earth: Azkaban, the wizarding prison (PA3, Sch1). Azkaban is a terrible place. Prisoners were for years guarded by Dementors, horrible creatures who have been described as "sightless soul-sucking fiends." They feed on positive emotions, which means that after spending much time in the prison, the prisoners lose all hope and all good feelings and thoughts. They are forced to relive the darkest, most horrible memories of their lives. Some go mad, others simply despair and die (GF27). During Voldemort's return to power, however, the dementors allowed the escape of a number of Death Eaters (OP25) and then eventually deserted Azkaban altogether to ally themselves with Voldemort (HBP1). After Voldemort's defeat, new Minister for MagicKingsley Shacklebolt saw to it that Dementors would no longer be involved in guarding Azkaban, which despite making the atmosphere more plesant on the island, likely prevents a number of problems as well (BLC).

Location

"Sirius did not have to pass through Little Whinging to get from Azkaban to Hogwarts; he just wanted to."      -- J.K.Rowling (Sch1) Because of a comment by Sirius Black describing his escape from Azkaban, some have conjectured that the island must really located farther south than the North Sea location Rowling has described. Sirius does state quite clearly that he swam to shore, then visited Privet Drive before heading north to Hogwarts (PA22). Privet Drive is in Surrey, which is south of London. Since it is only logical to assume that Sirius would have swum to the nearest point on the mainland to Azkaban, that point must have been somewhere in the English Channel between England and France. Several fans have tried to come up with alternative locations which fit the bill. Rowling, however, is very clear on the subject. Azkaban is far to the north, and Sirius chose to travel to Hogwarts with a detour through Surrey. That seems odd to us Muggles, but wizard ideas of "shortest route" don't necessarily use the kind of geometry we Muggles take for granted. The Knight Bus, for example, seems to move between destinations almost in alphabetical order. This odd logic is similar to that which makes Transfiguration simpler when the two items have similar names (guinea pigs and guinea fowl). Therefore we can't necessarily take Sirius' offhand analysis of his trip and superimpose it over a Muggle map to figure out where Azkaban is (Sch1)

Break-Outs

We know of at least four break-outs from Azkaban. We have been led to believe that these are the only times this has ever taken place, as Ron states clearly after Sirius Black's break-out that "no one's ever done it before" (PA5). While this isn't quite true, it does seem likely that the break-outs we've heard of are all that exist.
• Barty Crouch Jr. - c. Winter 1982
  "About a year" after he was brought in, Crouch's parents hatched a plan in which he would trade places with his dying mother. Barty Crouch Sr., at the time the Head of the Department of Magical Law Enforcement, led Mrs. Crouch into Azkaban to visit. While there, she and Barty Jr. each drank Polyjuice Potion to take on each other's appearance, and Crouch Jr. departed with his father while his mother was left in his place to die (GF27, GF33).
• Sirius Black - July 1993
  Cornelius Fudge came to visit Azkaban this summer, and gave a copy of his Daily Prophet to Sirius. On the front page, though, was a picture of the Weasley family that included Ron's pet rat, Scabbers. What Sirius alone knew was that Scabbers was actually an unregistered Animagus - Peter Pettigrew - who was also a Death Eater. An unregistered Animagus himself, Sirius transformed into a dog when the Dementors brought him food; in this state he was able to slip past them undetected and swim to shore (PA19).
• Antonin Dolohov, Augustus Rookwood, Bellatrix Lestrange, Rodolphus Lestrange, Rabastan Lestrange, Mulciber, and four other Death Eaters - January 1996
  With the Ministry of Magic losing control of the Dementors, ten "high-security prisoners" were allowed to escape and re-join Voldemort (OP25; we know Mulciber, Rodolphus, and Rabastan are included as they were imprisoned in GF30, but fought in the Battle of the Department of Mysteries in OP35).
• Stan Shunpike, Lucius Malfoy, Travers, Yaxley, and other Death Eaters - Summer 1997
  Kingsley Shacklebolt is certain that there was "a mass breakout which the Ministry... hushed up," which explains why Voldemort has all of his Death Eaters despite many having been sent to Azkaban the previous spring (DH5).

Known Prisoners of Azkaban, Past & Present

• Black, Sirius
  Captured and sent to Azkaban without trial (GF27) for betraying James and Lily Potter to Voldemort and then blowing up Peter Pettigrew and a street full of Muggles, c. November 1981 (PA10). Escaped July 1993 (PA3).
• Crabbe
  Captured at the Battle of the Department of Mysteries, June 1996 (OP35, OP36). Presumably escaped in the mass breakout that summer (DH5).
• Cronk, Crispin
  Sent to Azkaban by the Ministry for keeping sphinxes in his back garden, despite repeated warnings (fw).
• Crouch Jr, Bartemius "Barty"
  Convicted of participating in the torture of the Longbottoms in 1981 and sentenced to a life term in Azkaban - the penalty for the use of the Cruciatus Curse as one of the three Unforgivable Curses (GF30). Escaped disguised as his mother, who died in his place a year later (GF35).
• Dolohov, Antonin
  Sentenced c. 1981 for torturing "countless Muggles and non-supporters of the Dark Lord" (GF30). Escaped during the mass breakout of January 1996 (OP25). He was sent back to Azkaban after his recapture June 1996 in the Battle of the Department of Mysteries (OP35, OP36), and escaped a second time that summer (DH5).
• Dumbledore, Percival
  Sent to Azkaban after savagely attacking three Muggle children (DH2), in retaliation for their attack on his daughter, Ariana (DH28).
• Fletcher, Mundungus
  Sent to Azkaban in March 1997 for impersonating an Inferius during an attempted burglary (HBP21).
• Gaunt, Marvolo
  Sent to Azkaban c. 1944 for attacking Ministry officials attempting to arrest his son. He was sentenced to six months (HBP10).
• Gaunt, Morfin
  Sent to Azkaban for three years c. 1944 for the hexing of Tom Riddle and resisting arrest. After he returned from Azkaban, he was implicated in the deaths of the three inhabitants of the Riddle House. He readily confessed to the killings, but was actually innocent. His nephew, the son of Tom Riddle and Morfin's sister Merope, actually committed the murders and planted false memories in Morfin's mind. He died in Azkaban, firmly believing in his own guilt (HBP17).
• Hagrid, Rubeus
  Held for some weeks without trial for the basilisk attacks on Hogwarts students in the spring of 1993 (CS14), then released when his innocence was proven (CS18).
• Jugson
  Captured after the Battle of the Department of Mysteries, June 1996 (OP35, OP36). Presumably escaped in the mass breakout that summer (DH5).
• Karkaroff, Igor
  Captured by Alastor Moody after six months' pursuit, but released c. 1981 when he cooperated with the Ministry and gave them the names of other Death Eaters (GF30).
• Lestrange, Bellatrix Black
  Sentenced to life imprisonment in Azkaban for use of the Cruciatus Curse on the Longbottoms (GF30) c. 1981; escaped during the mass breakout of January 1996 (OP25).
• Lestrange, Rabastan
  Sentenced to life imprisonment in Azkaban for use of the Cruciatus Curse on the Longbottoms c. 1981 (GF30); escaped during the mass breakout of January 1996 (see note under 'Break-Outs'). Sent back to Azkaban after his recapture June 1996 in the Department of Mysteries (OP35, OP36) and presumably escaped a second time that summer (DH5).
• Lestrange, Rodolphus
  Sentenced to life imprisonment in Azkaban for use of the Cruciatus Curse on the Longbottoms c. 1981 (GF30); escaped during the mass breakout of January 1996 (see note under 'Break-Outs'). Sent back to Azkaban after his recapture June 1996 in the Department of Mysteries (OP35, OP36) and presumably escaped a second time that summer (DH5).
• Lovegood, Xenophilius
  Arrested in essence for being a nuisance, printing editions of the Quibbler that supported Harry Potter in defiance of the Voldemort-controlled Ministry, summoning authorities for made-up reasons after they arrested Luna, and then nearly blowing them up when he summoned authorities again to try to have Harry captured (DH21, DH22).
• Macnair, Walden
  Captured after the Battle of the Department of Mysteries, June 1996 (OP35, OP36). Escaped that summer in a mass break-out from Azkaban (DH5).
• Malfoy, Lucius
  Captured after the Battle of the Department of Mysteries, June 1996 (OP35, OP36). Escaped that summer in a mass break-out from Azkaban (DH5).
• McTavish, Tarquin
  Imprisoned for trapping his Muggle neighbor inside a kettle (JKR).
• Mulciber
  Captured by the Ministry c. 1981 or earlier (GF30); escaped during the mass breakout of January 1996 (see note under 'Break-Outs'). Sent back to Azkaban after his recapture June 1996 in the Department of Mysteries (OP35, OP36) and presumably escaped a second time that summer (DH5).
• Nott
  Captured after the Battle of the Department of Mysteries, June 1996 (OP35, OP36). Escaped that summer in a mass break-out from Azkaban (DH5).
• Podmore, Sturgis
  Sentenced to six months in Azkaban in late 1995 after being caught in an attempt to break into a restricted area at the Ministry of Magic (OP14).
• Rookwood, Augustus
  Turned in to the Ministry of Magic by Igor Karkaroff c. 1981, having been a spy for Voldemort in the Department of Mysteries (GF30). Escaped during the mass breakout of January 1996 (OP25). Sent back to Azkaban after his recapture in June 1996 in the Battle of the Department of Mysteries (OP35, OP36) and escaped a second time that summer (DH5).
• Shunpike, Stan
  Arrested after making jokes about Death Eaters and sent to Azkaban, autumn 1996 (HBP11). Stan was kept there by the Ministry mostly to show that they were doing something about the Death Eater threat, even though he was innocent (HBP16). He escaped the next summer and, most likely under the Imperius Curse, joined the Death Eaters in attempting to capture Harry (DH5).
• Travers
  Captured by the Ministry c. 1981 or earlier (GF30). It is unknown whether he escaped as part of the mass break-out of 1996; if so, he was re-apprehended. Either way he certainly escaped Azkaban in the summer of 1997, because according to Kingsley Shacklebolt he was "supposed to be inside" (DH5).
• Dolores Umbridge
  Following the defeat of Voldemort, Umbridge was "arrested, interrogated, and imprisoned for crimes against Muggleborns" (BLC).
• Yaxley
  Arrested for breaking into Hogwarts in 1997; he almost certainly escaped in the mass breakout that summer.
  
  Through a little deductive logic we can determine that Yaxley was the unnamed Death Eater who Harry Stunned atop the Astronomy Tower immediately following Dumbledore's death. When Harry ran into Yaxley at the Ministry, he recognized him as a Death Eater who had been present that night(DH12); meanwhile, the only people on the tower when Dumbledore died were Dumbledore, Harry, Snape, Draco Malfoy, Fenrir Greyback, the Carrows, and the "brutal-faced" Death Eater (HBP27) who Harry Stunned (HBP28). Therefore, this Death Eater must have been Yaxley - and Rufus Scrimgeour later told Harry that the Ministry had discovered that Death Eater, and presumably arrested him (HBP30). 

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Source : http://www.hplex.info/wizworld/places/azkaban.html