Situated at small distance from the Agora and the port of the ancient city of Kos, the Altar of Dionysus is a good example of a Π-shaped altar of the Hellenistic period. The part preserved is dated in the 2nd century b.C., a period of flourishing for Kos, as it is evidenced by numerous architectural remains. It is thought that the altar was sponsored by the king of Pergamos, an ally city of Kos. The monument was destroyed during the strong earthquake of 142 b.C. After the earthquake the residents of the area started gradually to restore it.

Later on, during the Byzantine period, the altar had been abandoned and when the Knights of the Order of St. John conquered the island, they used some architectural parts for the construction of the castle.

The frieze of the altar is dated back to the end of the 2nd century b.C. and now it is housed in the castle. It presents scenes from the Amazon war and a troupe of Dionysus, with maenads and satyrs.

Only a rectangular groundwork is well preserved today, along with the uphill entrance and a vertical stone wall.