The Baths of Trajan built over the Domus Aurea

Emperor Domitian began the project around 96 AD, and it was completed by Trajan in 109 AD. To create a flat platform, the architects used the ruins of Nero's Domus Aurea as a cryptoporticus to level the ground and support a platform built over it upon which the baths were constructed.

The complex rested on a northeast–southwest axis, which was off axis by about 30° with the nearby Baths of Titus. It is suggested that this unorthodox orientation was chosen by the architects to reduce the bathers' exposure to the wind, while also maximising exposure to the sun.

The bath complex was an immense 330 by 340 metre block designed for the Roman leisure experience. A visitor would enter through a vestibule on the northeast side and proceed to the natatio, a large open-air swimming pool surrounded by colonnades. They would then move through flanking wings toward the hot and cold rooms, completing the experience back where they began.

The southwest corner exedra, with its two stories of niches, still survives today. The complex also featured a system of subterranean passageways and structures used by slaves and workers to service and maintain the facilities, as well as a massive underground cistern that stored up to 8 million litres of water.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons