The Fortress Study Group actively encourages the research, study and recording of fortifications of all periods and publishes FORT, an annual journal, which contains articles about the theory, development, practice, conservation, adaptation and reuse of fortifications and military architecture. The society also publishes Casemate, a well-established magazine which appears three times a year.

The group actively co-operates with other national and international societies with similar interests in other countries throughout the world.  It is a member of the International Castles Institute (IBI) and a founding member of the International Fortress Council (IFC).

The Fortress Study Group owns a library of books and other reference material which is located at Fort Nelson and is available to members by arrangement.

Every year the FSG organises an AGM and Conference weekend in the UK, which includes trips to local sites of interest to the group and lectures on aspects of fortification. Previous conferences have been held in Alderney, Chatham, Cork, Cornwall, Dover, Edinburgh, Harwich, Hull, Inverness, London, Milford Haven, Newcastle, Orkney, Pembroke, Plymouth, Portland, Portsmouth, Ulster and the Isle of Wight.

The FSG has organised two symposia entitled ‘Fortifications at Risk’ to explore ideas on how the derelict fortifications of the 19th and 20th centuries in Britain might be protected and preserved. These have highlighted concerns at the number of 19th and 20th century fortifications that have become derelict or have been destroyed. Interested parties met to discuss the preservation of these structures, and imaginative ways in which they might be re-used. Speakers included prominent British and international figures in the heritage world. The second symposium in 2013 emphasised the practical aspects of how such fortifications can be re-used without these structures losing their historical integrity.

There is an annual Members Day, where members are encouraged to speak on aspects of fortification that they have visited, and a Study Day to a site of special interest. There is also an annual Study Tour to examine fortifications in other countries. Countries visited include Belgium, Bermuda, the Czech Republic, France, Gibraltar, Italy, Malta, the Netherlands, Poland, Portugal, Rhodes, Slovenia, and many other locations.