Falak-ol-Aflak Castle or Shapour Khast Gate belongs to Sasanid Era and it is located at the center of Khorram Abad, on a hill overlooking Khorram Abad. The Sasanids built a city known as Shapoor Khast about the location of the current city of Khorram Abad and later it was ruined. About 7th century A.H., the present Khorram Abad was built instead. Therefore, it is supposed that Falak-ol-Aflak Castle is that Shapoor Khast Gate that was built in Sasanid Era with state and military use. After constructing Khorram Abad, this castle was also known as Khorram Abad Castle. Falak ol-Aflak is the title used for this castle at Qajar Era and today it is also known as Falak-ol-Aflak Castle. Constructing this building on top of rocky cliffs completely overlooking the ancient valley of Khorram Abad as well as Golestan Spring flowing from the north skirts of the hill where Falak-ol-Aflak Castle is constructed, are some principle features of this castle. The plan of Falak-ol-Aflak Castle is an irregular octagon with a relative area of 5300m2 including 8 towers, 2 courtyards and 300 turrets. Materials like adobe, red and large bricks, stone as well as gypsum and lime mortar are used in the construction of this castle. The current architecture of the castle indicates abundant extensions and development made during various eras most of which belong to Safavid to Qajar Era. Currently, Archaeological Museum, Museum of Anthropology, the laboratory for repairing objects, cultural products shopping center as well as the classic tea shop are present in this historic-cultural complex. Falak ol-Aflaak Castle has been registered as a national monument of Iran.