The Roman Baths at Zappeion cover a surface area 21 metres wide between two strong, well-built walls, which incorporate earlier architectural structures. The bathhouse, which extends towards the National Gardens in the east and below Amalias Avenue on the west, included two chambers with hypocausts, two praefurnia (furnace chambers), and nine reservoirs.

The largest chamber had fifteen cylindrical and rectangular hypocaust columns, and dividing walls. This was the caldarium, or hot bath chamber. The tepidarium, or warm bath chamber, to the north, was a long room built on seventeen marble funerary columns instead of hypocaust columns. The praefurnia were linked to the caldarium by vaulted underground passages. The hot air was channelled into three small reservoirs. Vertical openings in the walls of these reservoirs provided ventilation and allowed for the air to heat the walls. A large, rectangular, and carefully constructed reservoir with a thick layer of hydraulic mortar on the inside and marble plaques on the outside belongs to this building phase. The reservoir supplied water for two marble basins discovered in situ.


In the fifth-sixth century AD (second building phase), the fourth-century hypocaust chambers were repaired and reused, and four more reservoirs with tiled floors were added. One of the reservoirs was built underground. It had a vaulted roof with an open inlet for water and was paved with tiles. Rough sketches of humans, fish, birds, and crosses, possible traces of the reservoir's use as a refuge or martyrion, decorate its north wall. In the Byzantine period, terracotta pithoi (storage jars) for grains were built into the floor of the various chambers. Some of these pithoi have been moved to the site's south section.