The square base is composed of two layers -- an outer marble-faced travertine wall and an inner brick wall. The outer wall stands 10-12 meters high and stretches 85 meters long on each side. The inner brick wall was bonded with the brick walls of a series of barrel-vaulted chambers radiating from the central drum.1 The modern entrance is located on the southern face.
The central drum within the square base stands 31 meters high from the ground. The drum and the central tower at the top have been severely damaged over time. The drum has an exterior made of opus quadratum of tufa with courses of travertine blocks in some places for extra support.2
1 S. Rowland Pierce, “The Mausoleum of Hadrian and the Pons Aelius.” The Journal of Roman Studies, Vol. 15 (1925), 82.
2 Ibid., 85.