Alamut Castle stands high in the Alborz northeast of Qazvin, on the site associated with Hassan-i Sabbah and the Nizari Ismaili state. The ruins are fragmentary and partly reconstructed, but the setting is the point: steep rock, mountain villages, and a view that explains why this place became a fortress.
Allow a full day from Qazvin. The drive is winding, and the surrounding Alamut valley deserves time rather than a quick photograph at the top.