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'Upon the top of this building of Augustus, Vespasian built his palace in A.D. 70, not only using the walls of the older palace as a support for his own, but filling the chambers of the earlier building entirely up with earth, so that they became a solid massive foundation. The ruins which we visit are thus for the most part those of the palace of Vespasian, but from one of its halls we can descend into rooms underneath excavated from the palace of Augustus.'
Augustus Hare, Walks in Rome, London: George Allen, c1892, volume I, page 301