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. His first book, The Castles of the Assassins, concentrating on Rudbar and the Alamutinfo-icon Valley, was published in 1965 and re-published in Iran and the United States in recent years. His last work, Eagle's Nest: Ismaili Castles in Iran and Syria, is an expanded and revised study on the Ismaili castles in Iran and Syria, published in 2005 by The Institute of Ismaili Studies in association with I. B. Tauris.

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.