Between 2000 and 2014, Marshall was chair/secretary of the Castle Studies Group and is Comité Permanent of the Colloques Château Gaillard, a biannual conference for castellologists.
Marshall completed a PhD at the University of Nottingham in 2006, titled Magna Turris: A study of the development, planning and use of social space in donjons of the eleventh and twelfth centuries located in the geographical territories of the Norman and Angevin kings of England.
Marshall was one of several researchers to apply this approach to great towers (keeps), alongside Philip Dixon and Sandy Heslop.
[4][5] Dixon and Marshall studied Hedingham Castle's keep, proposing to a "radical re-assessment" of the building as a ceremonial rather than domestic space.
[2] The results were documented in the book Guardian of the Trent: the Story of Newark Castle, which won an Alan Ball Local History Award in 1998.