A description of the tower is given in the "Memoirs of the Academy of Inscription," quoted by Bertrand in his "History of Boulogne," as follows: "The form of this monument, one of the most striking erected by the Romans, was octagon. It was entirely abolished about a hundred years ago, but, fortunately, a drawing of it, made when the lighthouse was still perfect, is still in existence, and has been exhibited to the Academy by the learned Father Lequien, a Dominican monk, native of Boulogne. Each of its sides, according to Bucherius, measured 24 to 25 feet, so that its circumference was about 200, and its diameter 66 feet. It contained twelve entablatures, or species of galleries, on the outside, including that on the ground floor. Each gallery projected a foot and a half further than the one above it, and consequently their size diminished with each succeeding gallery. On the top fires were lighted to serve as a beacon to vessels at sea. A solid foundation was formed, not only under the lighthouse, but for some distance beyond the external walls. It was constructed of stones and bricks in the following manner: first were seen three layers of stones, found on the coast, of iron grey colour, then two layers of yellow stone of a softer nature, and upon these two rows of hard red bricks, two inches thick, and a foot and a half long, and a little more than a foot broad" ("Bertrand's History of Boulogne," pp. 13, 14). "Caligula's tower was built on the north-eastern cliffs, about half a mile from the sea, but within the suburbs of Boulogne. The constant encroachment of the tide had reduced that distance to 400 feet in 1544, when Boulogne was captured, and fortifications built around the tower by the English troops. Still, however, the merciless waves rushed onward to the coast, undermining the cliffs more and more, until at length, on July 29th, 1644, Caligula's tower fell headlong with a crash into the sea. "Passengers from Folkestone to Boulogne gaze with reverence or curiosity on the Calvary on the northeastern cliffs, which fishermen salute with uncovered heads when sailing out to reap the harvest of the sea. Close to the Calvary there is a mass of ruins overhanging the cliff, which is all that remains of the fortifications built round Caligula's tower by the English conquerors. The tower itself once stood over the site occupied by the Hotel du Pavillion et des Bains de Mer, opposite the place for sea bathing" ("Bertrand's History of Boulogne," pp. 15, 16).