Public baths were used for as recreational and educational means for the citizens of Rome. An entire public bath complex could be 50 acres large with the actual bathing area taking up 5 acres of that. The bath complex consisted of bath buildings in the center with other facilities on either side that consisted of exercise rooms, dressing rooms, shops, and public toilets. Also on the complex grounds were libraries, stadiums, painting galleries, auditoriums, and large water reservoirs. The bath center was engineered so that the waste water from the public baths would then be used to flush out the public toilet's sewage.
The bathing centers were all grouped into the center of the complex together so that they Romans could more efficiently keep them heated with the furnaces they had underneath the area. They had hot baths and they also had cool baths.
Bath complexes were constructed using all of the typical Roman aspects. They used a concrete and brick structure and then used colorful marbles and mosaics to decorate it lavishly. They used their infamous round arches and high groin and barrel vaults to maximize the amount of space without having to worry about crowding the place with a bunch of supporting structures. The architects were building to suit incredibly high volumes of Romans. They used the groin vaults in the center with barrel vaults on either side. This allows for large windows to be put to use. Bath houses depended quite largely on windows for natural light. They could not be used at night.