The former military tower in which Buck Mulligan and Stephen Dedalus are staying has living quarters on the second level and battlements on the third. A steep stone passage of "dark winding stairs" connects these two stories, from whose "stairhead" (shown in the center of the photograph) first Mulligan and then Stephen emerge into the open air.
As the cutaway diagram shows, these so-called 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 the diagram calls the "gun platform," and Joyce calls the "gunrest." Part was borne by the raised step next to the high wall that Joyce 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 in the nineteenth century, after the towers became militarily obsolete.