As was explained to me by one of the re-enactors within the fort, earth was piled up against the inside of the walls, supported by wooden structures. This served as a rampart, atop which cannon were mounted. How many cannon, and precisely what kind, seem to be unknown. Really large cannon weren't ever mounted, however, because nobody involved with building the fort thought that the French, Fort Frederick's imagined attackers, would be dragging anything bigger than the puniest of guns all the way to Fort Frederick.

You will perhaps note that this is the inside of a pointy starfort corner, which one does not see all that often.