Labour Service") ceased work on Wewelsburg, the Reichsarbeitsdienst (the "RAD", "Reich Labour Service") carried out modifications to the castle; but in 1938, the RAD was relocated to the "Westwall" (Siegfried Line). Between 1939 and 1943, prisoners from the Sachsenhausen and Niederhagen concentration camps were used as labourers to perform much of the construction work on Wewelsburg, under the design of architect Hermann Bartels. However, by a decree of 13 January 1943, all building projects which were unimportant for the war – including the Wewelsburg – had to be stopped. In 1938, after Reichskristallnacht, 17 Jews from Salzkotten, ten kilometers (about six miles) distant, were held in the dungeon of the Wewelsburg before transportation to the Buchenwald concentration camp. In the middle of the 1930s, Himmler had a private safe mounted in the basement of the west tower. Only the commandant of the castle knew about it. The whereabouts of its content after the Second World War is unclear. Meetings of SS-Leaders Swearing-in ceremonies were planned at the castle. Meetings of SS-Gruppenführer (equivalent to lieutenant-generals) at so called "spring conferences" were planned since 1939. Some talks probably took place at Wewelsburg Castle; the only documented Gruppenführers''' meeting was held from 12 to 15 June 1941 – one week before the beginning of Operation Barbarossa. The highest ranking SS-officers, who planned the SS operation in the Soviet Union or who were intended to be used for the operation, were called up by Himmler. Concrete decisions were not made. The meeting's purpose was the ideological preparation of the attendant SS leaders for the campaign. Another source mentions three or four ceremonies a year of SS leaders which took place at the castle. Towards the end of the war Himmler ordered that Wewelsburg castle should become the Reichshaus der SS-Gruppenführer (Reich-House of the SS-Gruppenführer). Death's head rings In 1938, Himmler ordered the return of all death's head rings (German: Totenkopfringe) of dead SS-men and officers. They were to be stored in a chest in the castle. This was to symbolize the ongoing membership of the deceased in the SS-Order. The whereabouts of the approximately 11,500 rings after the Second World War is unknown. SS plans Himmler's plans included making it the "center of the new world" ("Zentrum der neuen Welt") following the "final victory". The monumental estate was never realized; only detailed plans and models exist. The installation of a 15 to 18-meter-high wall