In the centre of Athens, southeast of the National Garden, excavations have brought to light a large bath complex of the late Roman period. In this area next to the Ilissos river, incorporated into the city of Athens during the reign of Hadrian in the 2nd century CE, sanctuaries, public buildings and luxurious private villas were built. The lush landscape of the area, one of the most beautiful suburbs of the city, offered the Athenians an idyllic, refreshing environment suitable for strolls and discussions.

The so-called Bath of Zappeion is an excellently preserved bathhouse (balneum), a public bath for the inhabitants of the city. A large part of it remains unexplored as it extends east and west beneath the present urban fabric. The construction of the bath dates back to the end of the 3rd or the beginning of the 4th century CE, after the raid of the Heruli on Athens. Among the architectural remnants identified are two hypocaust rooms, two heating furnaces (praefurnium) and nine cisterns.

The large space identified as the caldarium, or hot bath chamber, has 15 hypocaust pilasters that supported the floor and partition walls for the underground communication of the hot air with the prefurnia. The tepidarium, the room for the lukewarm bath, is located to the north. Its floor was supported by 17 small marble columns, whose initial use was funerary. The air was heated in three small tanks and directed into the hypocaust rooms through arched passages. Vertical openings in the walls of the tanks through which the hot air passed served to heat the walls. Finally, the excavations revealed a rectangular tank lined with marble slabs on the outside and a thick layer of hydraulic mortar on the inside, which channelled water into two marble basins.

In the 5th and 6th centuries, extensive repair and extension works were carried out after the destruction of the bath, probably related to the raid of Alaric (396 CE). The hypocaust chambers and the small tanks were repaired and the bathhouse was equipped with four new paved cisterns.

In one of the cisterns built in this second phase of the bathhouse, makeshift wall paintings were found on the northern wall. Researchers attributed them to the use of the site in the following centuries as a shelter or martyrdom, a place dedicated to the memory of a Christian who was killed for defending his religious faith. Later, during Byzantine times, the space was used for storage, as large pithoi (storage jars) were wedged into the floors and built-in storage containers were placed into the rooms. In the late Byzantine period, two kilns were constructed, and, after the foundation of the Greek state, the area was covered during the landscaping of the National Garden.