The Mausoleum was built by Augustus as his own family tomb, looks back to the Etruscan tumulus for its basic form, though it broke the long-standing rule forbidding burial inside the city. Like the later Mausoleum of Hadrian, it had as its base a huge cylinder, 88 m (290 ft) in diameter. The outer wall was constructed of concrete, faced with travertine opus quadratum. Behind this, a complex system of four circumferential and numerous radial walls, similarly constructed of concrete but now mostly faced with opus reticulatum, divided the interior into concentric compartments, though, apart from the sepulchral chamber and the passages leading to it, these compartments were simply filled with earth.