A castle in Westphalia near Paderborn in Germany, acquired by the Waffen SS in 1934 and rebuilt throughout World War II under Heinrich Himmler's orders. During the 17th century the castle was the secondary residence of the prince bishops of Paderborn, but its location caught Himmler, Karl Maria Wiligut, and the other occultists in the Ahnenerbe SS's eyes: they believed it to be the "Mittelpunkt der Welt" (middlepoint of the world), a locus for great occult and mystical power. Wiligut, eccentric self-proclaimed clairvoyant that he was, believed that the castle would become a Nazi bastion against an invading barbarian army from the East, so Himmler then went on to make it the center for the pseudoscientific ideology of the Nazi regime, as well as a memorial to commemorate the dead SS Leaders. It became something of a "Camelot" for Himmler's new order of knights, the Schuztaffel.
Restoration work by the SS began in 1934, using slave labor imported from various concentration camps all over Germany. By 1939, the north tower, which was struck by lightning in 1815 was rebuilt, and construction proceeded at a furious pace. By 1941 the number of slaves at Wewelsburg was so great that a small concentration camp known as Niederhagen was created outside the castle. Of the 3900 prisoners there, at least 1285 are known to have died, killed by undernourishment, hard work, poor hygeine, arbitrary punishment, and deliberate murder. Most of these poor inmates were German Jews and Jehovah's Witnesses, as well as captured Soviet prisoners of war.
In the newly rebuilt North Tower, where the old chapel of the prince bishops was once located, the "Hall of the Leaders" was created. The room contained a large round table of oak, where Himmler and twelve of his most senior SS-Obergruppenführers (Lieutenant-General, ranked right below Himmler's Reichsführer rank) sat on high-backed pigskin chairs. The parallels with King Arthur and the Knights of the Round Table as well as with Jesus Christ and his twelve disciples were intentional. Here these leaders of the SS would be made to sit with their Grand Master Himmler, contemplating and meditating for hours at a time.
Directly below this hall was built a crypt, called the "Hall of the Dead". This is a large circular chamber with twelve low platforms against its wall. A circular depression with three steps down is in the center of this room, which contained an altar, upon which burned a sacred fire. It is thought that Himmler and some of his initiates used this chamber to perform dark rituals associated with the Black Sun of the Vril Society, some of which involved human sacrifice. The sign of the Black Sun is found all over the castle (yet another symbol, along with the swastika, whose display is today banned in Germany) Another theory about the use of this chamber was that it was to be used in funeral ceremonies for high-ranking SS officers, whose crests and urns of ashes were to be placed on the platforms.
All of the knights of the "round table" were given private quarters within the castle, and Himmler's in particular is quite interesting. His chambers were dedicated to the 10th century Saxon king Heinrich I (also known as Henry the Fowler) and were decorated in period fashion. It is thought that Himmler believed himself to be the reincarnation of the ancient Saxon monarch, or perhaps believed that he was an honorary member of that royal bloodline, with the king's spirit living in him. In any case, at midnight every July 2nd, on the anniversary of Henry the Fowler's death, Himmler would apparently commune with the spirit of the dead monarch.
As Germany was being overrun by Allied forces near the end of World War II, Himmler ordered the castle's destruction by explosives on March 31, 1945, to prevent its capture. However, only three days later, before the order could be carried out, American troops arrived and liberated the last surviving prisoners at Niederhagen and captured the castle.
A grim picture of the exterior of the castle can be found here: http://www.schwartzeorden.org/gralsburg.htm
Other pictures of the castle, including pictures of the Hall of the Dead with the Black Sun insignia and swastika can be found here:
http://www.schwartzeorden.org/wewelsburg.htm
These link to what appears to be the website of a neo-Nazi Satanic cult of some sort (the text is German, and I make no pretentions to being able to read it). However, the name "schwartzeorden" I vaguely recognize as meaning "The Black Order", which is yet another appelation of the SS... Wewelsberg had after the war become a magnet for such practitioners of Satanism and the dark arts, probably due to Himmler's own proclivities for such things. Michael Aquino of the Temple of Set is known to have performed rites there, among many others, so much so that occult researcher Peter Levenda described it as the "Satanic Vatican" in his book cited below.
The castle itself is apparently one of the major sources of inspiration for the Wolfenstein games, such as Return to Castle Wolfenstein and Spear of Destiny with all their allusions to Nazi demonology and black magic.
Sources:
Unholy Alliance: A History of Nazi Involvement with the Occult, Peter Levenda and Norman Mailer.
The Messianic Legacy, Michael Baigent, Richard Leigh, and Henry Lincoln.
"Dragon's Hoard - Heinrich Himmler", http://www.whitedragon.org.uk/articles/himmler.htm
"Memorial museums for the victims of NS in Germany", http://www.topographie.de/gedenkstaettenforum/uebersicht/e/d_31.htm
"The Nazi UFO Enigma", http://www.fsreview.net/spi/nazinth.htm