Over the course of several pages in Telemachus the reader receives various details about the living area inside the Martello tower at Sandycove. The tower is currently maintained as a museum which attempts to recreate the living conditions that Joyce experienced in 1904. Photographs of the interior can give some sense of the setting evoked in the novel.
"In the gloomy domed livingroom of the tower Buck Mulligan's gowned form moved briskly about the hearth to and fro, hiding and revealing its yellow glow. Two shafts of soft daylight fell across the flagged floor from the high barbacans." A stove presently vents into the fireplace, but it seems clear from the narrative that Mulligan is cooking over the open fire of the “hearth,” since the yellow glow of the coals is visible and "a cloud of coalsmoke" fills the room, choking its inhabitants.
Two shafts of soft daylight are indeed visible in the photograph at right, slanting into the room from angled wells that reach up to openings in the thick stone walls that the narrative refers to as “high barbacans." In Circe the Black Mass that Mulligan has mockingly evoked on the top of the tower is reenacted in this room below, on the second floor, and the hearthstone becomes an altarstone: "From the high barbacans of the tower two shafts of light fall on the smokepalled altarstone."
Barbicans (from French barbacane and Provençal barbacana, hence perhaps Joyce's spelling) can be towers or other defensive outer works of a castle, often near a gate or drawbridge. But as Marc Therre observes in a personal communication, the OED lists the obsolete meaning of "A loophole in the wall of a castle." Therre notes that "barbacan," spelled this way and carrying this sense, is used both in William Beckford's Gothic novel Vathek (1816) and in Carlo Botta's History of the War of the Independence of the United States of America (1826, translated from the 1809 Italian original).
In a corner of the room opposite to the fireplace, a hammock has been slung to recreate Trench’s sleeping conditions, mentioned twice in Telemachus: "A tall figure rose from the hammock where it had been sitting"; “he took his soft grey hat from the holdfast of the hammock.” (The word holdfast apparently refers to a bolt anchored in the stone wall.) Joyce's bed was probably to the left, between Trench's hammock and the fireplace.
In Trench’s nightmare, a black panther was crouching on the hearth, waiting to spring, and the Englishman fired his revolver in the direction of the fireplace, quite close to Joyce's bed. The curators of the museum have installed a kitschy statue to play the part of the beast.
The "heavy door" which Mulligan cries out to have opened, itself accessed by two "inner doors," is different from the opening that leads via steep stone stairs to the roof. It leads outside, via some metal stairs connecting the second-story livingroom to the ground. Joyce calls this staircase a "ladder." In 1904, it afforded the only ingress to the tower, and the large key mentioned repeatedly in the narrative allowed the iron door at the top to be locked. Today, an opening has been cut into the tower on the ground floor as an entrance to the museum.
When breakfast is finished, the three men exit the tower through this exterior door on their way to the swimming hole.