Schloss Hartheim, also known as Hartheim Castle, is a castle at Alkoven in Upper Austria, some 14 kilometres (9 mi) from Linz, Austria. It was built by Jakob von Aspen in 1600, and it is a prominent Renaissance castle in the country. The building became notorious as one of the centers for the Nazi killing program known as Action T4, in which German citizens deemed mentally or physically unfit were systematically killed with poison gas. After 1942, these killings were extended to include Jews, Communists and others considered undesirable by the state. In this capacity it was known as the Hartheim Euthanasia Centre.