Nurmengard Castle

Nurmengard is a wizard prison, described as "a towering building" and "a grim fortress, jet black and forbidding".

Nuremgard was built on the orders of the Dark Wizard Gellert Grindelwald at the height of his power in order to hold his opponents. Grindelwald's slogan "For the Greater Good" is carved over the prison's entrance. It is unknown specifically what form of magical defenses the prison has, but it seems to have anti-Apparition wards, similar to Hogwarts, as well as high walls.

After a number of years terrorizing Europe, Grindelwald was confronted and defeated by his former best friend, Albus Dumbledore. Others say that during his duel with Dumbledore Grindelwald realized his fault and allowed himself to be defeated. After the duel he was incarcerated in his own prison to reflect on his actions in the top-most cell, or so the wizarding community thought, truthfully he willingly went to the prison to create Horcruxes and meditate on a new enemy that he had seen in a vision. The enemy was a boy named Tom.

In his quest for the Elder Wand, Lord Voldemort traveled to Nurmengard in 1998, using his ability to fly to break into Grindelwald's cell. After confronting him for information on the wand's location, Voldemort killed Grindelwald, and then flew away. It is however believed that Nurmengard itself as well as the Elder wand may be Horcruxes created by Grindelwald. As this being the case he is not truly dead.

Etymology
Nurmengard likely comes from Nuremberg, the prison used to hold infamous Nazi war criminals, and later the location of the infamous Nuremberg Trials.