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 baths were erected on the Oppius Hill, a southern extension of the Esquiline Hill. Built on a platform which had itself been built over Nero's Palace, the bath complex was immense by ancient Roman standards, covering an area of approximately 330 by 215 meters. The complex rested on a northeast-southwest axis, with the main building attached to the northeast wall. This was contrary to the more widely used north-south axis of many buildings in the vicinity. It is suggested that this unorthodox orientation was chosen by the architects to reduce the bathers' exposure to the wind, while also maximizing exposure to the sun.
Within the complex, the building was surrounded by a large grassy area. The baths themselves consisted of pools, including a tepidarium (warm area and presumably first room visited in the baths), a caldarium (hot pool and dry, sauna-like area), frigidarium (cool pools used after those previously mentioned), and also gymnasia and apodyteria (changing rooms). In addition to the facilities of the bath complex used by the public, there was a system of subterranean passageways and structures used by slaves and workers to service and maintain the facilities. Also underground, the massive cistern, surviving today as the Sette sale, the "seven rooms", stored much of the water used in the baths. It was capable of storing no less than 8 million liters. There were also several exedrae on the eastern and western sides of the building