The principal monuments of the Nizari Ismaili state, which also defined and defended its boundaries, were the exceptionally well-constructed and provisioned castles that dominated the surrounding valleys and countryside. These castles varied in size from the massive fortified complex built on the sides and the top of a spur of the Alburz Mountains at Girdkuh near Damgan to a cluster of smaller, independent fortified sites in Khurasan or the Ansariya Djebel in Syria. Sometimes three or four large castles were built at a strategic site, such as Firdaws, to protect the southwest flank of the Ismaili state.
Major Peter Willey was a leading international authority on Ismaili castles of Iran and Syria. He began his research in 1959 and his most recent visits with Adrianne Woodfine included the Damghan/Semnan area and Quhistan in1996 and 1997 and Syria in 1998.