were conducting the first trials of this method at Auschwitz, Sachsenhausen had already been the scene of "some gassings in conjunction with the development of gas vans". The prisoners were also used as a workforce, with a large task force of prisoners from the camp sent to work in the nearby brickworks to meet Albert Speer's vision of rebuilding Berlin. Camp layout In July 1936, the Esterwegen concentration camp and Columbia concentration camp were closed and those prisoners moved to the Oranienburg concentration camp. That summer, those prisoners began clearing an of triangular forested area. By 1937, the prisoners had erected prisoners' barracks and SS guards' quarters, and SS officers' families housing. The "protective custody" internment camp was laid out in an isosceles triangle with sides long. Tower A was at its central control point, linked to the SS troop camp outside along the central axis. The entire camp could be viewed by the SS command staff from Tower A. Initially in area, the camp eventually grew to cover . Designed by Bernhard Kuiper, Himmler called Sachsenhausen a "completely new concentration camp for the modern age, which can be extended at any time." In practice, however, extending the design proved impractical. There was an infirmary inside the southern angle of the perimeter and a camp prison within the eastern angle. There was also a camp kitchen and a camp laundry. The camp's capacity became inadequate and the camp was expanded in 1938 by a new rectangular area (the "small camp") northeast of the entrance gate and the perimeter wall was altered to enclose it. There was an additional area (Sonderlager) outside the main camp perimeter to the north; this consisted of two huts Sonderlager 'A' and 'B' built in 1941 for special prisoners that the regime wished to isolate. Neutral zone The neutral zone was located between the camp wall and the prisoners' camp. Between the zone and the wall was a trip wire, Cheval de frise, barbed-wire obstacles, an electrified barbed-wire fence, and a sentry path. Slave labour Sachsenhausen was the site of Operation Bernhard, one of the largest currency counterfeiting operations ever recorded. The Germans forced inmate artisans to produce forged American and British currency, as part of a plan to undermine the British and American economies, courtesy of Sicherheitsdienst (SD) chief Reinhard Heydrich. The Germans introduced fake British £5, £10, £20 and £50 notes into circulation in