Placed on the top of Oppian Hill (Colle Oppio), these thermal baths were built by Traiano in 109 AD on the project of Apollodoro di Damasco, the most famous architect of that era. It is known that these thermal baths, also used by women, were still in use during the IV and V century.The structure was on a surface of almost one square kilometre, on the south side of the hill, on a terrace that was partly on the Domus Aurea.
On the walking part of the thermal baths a large enclosure with an exedra included a green area, in the centre of which there was the actual thermal building with specular rooms on the side of a central axis. Various colossal ruins of the thermal complex can still be visible nowadays: of the external enclosure are still preserved the NE, SW and part of the SE exedrae, apart from the grand central hemicycle on the SW side which is still perfectly preserved on the lower level. The ruins of the NW exedra are preserved in the cellars of the buildings in Via in Selci 79b. Of the central body there is part of the structure inside the park : the exedra and the eastern gymnasium, the apse of the room on the South side and part of the walling of the basilica reused as a hunting lodge by the Brancaccio family.
Prior to the construction of the Baths, their location on the Oppian Hill was occupied by the ornate Palace of Nero (Domus Aurea). After Nero’s suicide, subsequent emperors Vespasian, Titus, and Domitian chose to build over his palace with other forms of architecture. Emperor Trajan covered up the last of the palace with a platform upon which the Baths were built. Because they served as a model for bath complexes built throughout the Roman world during the Imperial period, these Baths would come to be recognized as a highly notable example of early Imperial Roman architecture. The archaeological excavations of 1997 also led to the discovery of a large (about 10 square meters) frescoed bird’s-eye view of a walled port city, a unique survivor of such a subject, in a buried gallery or cryptoporticus beneath the baths, which predated their construction, but postdated Nero’s Domus Aurea.
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