One of the chief requirements to be able to call something a fort, is that some sort of fort must be present. In Fort Frederica's case, all that's left is a single, diminutive piece of what had been the King's Magazine, which also served as the fort's last-ditch defensive structure: A redoubt in English military parlance, which would have been called a keep in the days of castles.

Before us we see the sign, The Fort.