The novel's first action, "Buck Mulligan came from the stairhead," situates the reader on the roof of a small military tower in Sandycove, seven miles from Dublin on the southeastern shores of Dublin Bay. Mulligan then stands at the stairhead and calls "down the dark winding stairs" of a steep stone passage that connects the tower's topside battle station to the living quarters one floor below. Finally, he moves to the raised stone platform that once held a swivel cannon but now serves as his makeshift altar: "he came forward and mounted the round gunrest."
The British Martello towers usually mounted a single gun that swiveled around a central pivot. Part of the weight of the cannon was supported by the raised platform that Joyce calls the gunrest. Part was borne by a raised step next to the high wall that Joyce, later in Telemachus, calls a "parapet." As the photograph shows, flat iron rings embedded in both structures allowed the gun to rotate freely without wearing down the granite. The gun on this tower was removed when large battleships made the structures militarily obsolete.