The altar of Dionysos is situated in an enclosed area of Kos town, a few meters NE of the Casa Romana, at the same level as that of the Hellenistic Agora of Kos. It was built with white and gray-blue marble of the island and dates back to the middle of the second century B.C. -a time of booming activity and expansion for the city of Kos- and is typical of a Hellenistic type of Altar, bearing quite a few resemblances to the great altar of Pergamon. The Altar of Dionysos was damaged somewhat during the 142 A.D. earthquake and partially restored. Many parts were used by the Knights of St. John in the construction of the Neratzia Castle. The site was excavated by the Italians in the beginning of the 1930s but their discoveries were not fully disclosed or published. It did not attract much research until 1981 when the doctoral dissertation of archaeologist N. Stambolidis was published.
The Altar is composed of two basic parts: the main body in the shape of a rectangular parallelogram and a slanted plane (ramp) in the middle of its western side. The frieze of the altar with thirteen sculptured cornerstones represent mythical themes from the life of Dionysos and are displayed today in the inner surrounding wall of the Neratzia Castle‘s Museum.
A few meters NE of the Altar, the stone base of a temple of the same Hellenistic period was found with some epistyles and fluted drums.