The first peripteral temple dedicated to Poseidon Isthmios, an archaic temple that dates back to the 7th century, was erected at the sanctuary of Isthmia. There are another two significant temples of Poseidon, one in Sounio and another in Paestum, Italy.
The area of Isthmos held the Isthmian Games, one of the four Panhellenic Games of ancient Greece and the second to be held in the region of Corinth. Similar to the Nemean Games, the Isthmian were held the year before and the year after the Olympic Games, the second and fourth year of each Olympiad. The Games were dedicated to Melicertes, also known as Palaemon, and they were established by Sisyphus the legendary founder of Corinth.
The French Archaeological School carried out the first systematic investigation of the site in 1883. The findings of the archaeological investigation, that highlighted the ancient theater and the stadium, were published one year later.
Oscar Broneer carried out the first excavations at the site during the years 1952-1967. In August 1989, a new excavation was conducted to clarify Broneer’s announcements and to present the architectural elements and their use.
At present, the archaeological site of Isthmia contains remains of a late Mycenaean wall, a construction, 2 to 3.5 meters wide, of an uneven form built of ashlar poros and filled with earth. It dates to the 12th century B.C.
Only few remains of the ancient theater are preserved at present as it was literally deconstructed to supply the neighboring medieval fortress with construction material. The excavations have brought to light part of the skene and the cavea foundations, some segments of the west parodos (entrance), the foundation of the proscenium and some stones pertaining to the foundation of two pedestals.
The temple of Isthmia was dedicated to Poseidon and was first built during the archaic period, in 7th century B.C. In 470 B.C., it was destroyed and during the classical period it was replaced by a new, larger temple also destroyed by fire in 390 B.C. The outline of the theater attests its large dimensions. There are another two significant temples of Poseidon, one in Sounio and another in Paestum, Italy. The site that surrounded the temple of Poseidon at Isthmia presented great activity until the 3rd century A.D. It contained roman baths, theater and two temples.
Palaimonio, the roman temple, dedicated to the marine divinity called Palaemon, was also excavated by O. Broneer. It has the same orientation as the temple of Poseidon and it was 8.30 meters long, 7.70 meters wide and 1.85 meters high. It had a special precinct and three sacrificial pits on the east side.
The stadium of Isthmia was renown before the excavations. It was located 300 meters to the southeast of the temple of Poseidon. The starting line of the previous stadium was discovered on the southeast corner of the temple, where one can distinguish the traces of the starting balbis, 16 in total. The later stadium was established in a natural cavity and it was approximately 181.15 meters long. The athletes of the Isthmian Games competed in this stadium. According to mythology, a dolphin brought the body of young Melicertus out to the shore of Isthmia, where it was found by the king of Corinth Sisyphus. He named the boy Palaemon and instituted the Isthmian Games in his honour. The winners of the Games received a wreath of pine leaves. Since their inception, the Isthmians Games were held under the control of the Corinthians. When the town was destroyed by the Romans in 146 B.C. the Games continued but they were administered by the Sicyonians. Following the rebuilding of Corinth in 44 B.C. by Ceasar, the town recovered control until they were suppressed by Theodosius as a pagan ritual.