The castle is centred on a rectangular stone keep built around 1200 by the St John family. Fonmon was enlarged with the addition of wings in the later medieval period. One surviving tower may date from that period.
A tour of the castle takes in the hall, drawing room, kitchen, and library, and features wonderful Georgian decoration throughout, with extensive displays of fine furniture, porcelain, books, and artwork. The drawing room was once used as a meeting room by John Wesley, and the kitchen dates to about 1650.