Upon entering the drum is an entrance corridor (B in the floor plan) and a vestibule (C) with a niche in the back wall that held a colossal statue of Hadrian. The interior of the drum is mostly solid concrete except for a spiral ramp (D) leading up from the right side of the vestibule to the burial chamber that's located at the center of the drum. With the original marble lining removed, the travertine on the walls is now exposed.1
Along the ramp there are four vents in the ceiling (shown in the isometric view). The ramp lead to the square burial chamber with vaulted ceiling at the center of the drum (G). It measures 10.2 meters high and 8.5 meters on each side. On the noth, east, and west walls are niches that probably held ash urns.2 Above this chamber is another room, and above which rises the central tower.
1 Amanda Claridge, Rome: An Oxford Archaeological Guide, (Oxford: Oxford UP, 2010).
2 Boatwright, Hadrian and the City of Rome, 172.