The "caldarium" was a circular room with marble floors and topped by a dome of almost 36 meters diameter, close to the size of the Pantheon's dome. The weight of the dome was carried by just eight masonry pillars. Between them were glass windows that helped heat the large room (and reduced the weight of the walls). Its seven pools measured 9.5 m by 5 m (depth of 1 m). Only six of these remain. The seventh was replaced by a small apse during the restoration by Constantine.