The Altar of Poseidon is located on a rocky ground in the west end of Tekağaç Peninsula, where Didim is located. Since Poseidon was the God of Sea in Greek mythology, the temples or altars that are dedicated to Poseidon were built in places that look over the sea. Today, only the base part of this altar, which is 20 m long and 11 m wide, built perpendicular to the sea in the east-west direction, is partly visible. This altar that constitutes the southern border of Ionia was the place for offering sacrifices and gifts to Poseidon. According to the representation of Poseidon in the mythology, he strikes not only the seas, but also the lands with his trident. Today, there is a statue in Altınkum that describes Poseidon as this.

The cape where the altar and the lighthouse are located is within the military area.