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 Willey has also worked in the northern areas of Pakistan with a focus on Gilgit and Hunza. He has also investigated the abuse of women's rights in Turkey and the effects of the narcotics trade in Afghanistan for Anti-Slavery International. Major Willey lectured at Bristol University on Islamic Art and Culture.