Zerzevan Castle, also Zarzavan Castle or Samachi Castle, is a ruined castle located in Diyarbakır Province, Turkey. It was built by Romans in the 3rd century AD as a military base on the ancient trade route between Diyarbakır and Mardin. The castle was in use in the Byzantine era until the 7th century AD. One of the most important monuments remaining in the castle is the Mithraeum, which is a temple for a Roman mystery religion centered on the god Mithras.