The archaeological site occupies the level top of the Philerimos hill at the NW part of the island. This was the acropolis of ancient Ialysos, one of the three city-states of ancient Rhodes, leader of the initiative which resulted in the establishment of the ancient city of Rhodes in 408/407 BC. Of its buildings only the foundations of the temple of Athena Polias and Zeus Polieus survive, among structures of later periods. It was a hexastyle temple with a tetrastyle portico in antis. The interior contained a Corinthian colonnade framing the cult statue on three sides. It dates from the 4th c. BC, although worship on the plateau goes back to at least the 9th c. BC. In the 5th-6th c. AD part of the temple served as foundation for a large three-aisled Early Christian basilica, whose baptismal font is still preserved in good condition.
On a shelf of the steep SW slope of the hill, about 30 m. lower from the level of the south side of the plateau, a stone staircase leads to the Doric Fountain. Built in the 4th c. BC, this is one of the most graceful ancient monuments of the Dodecanese Islands. It is a sandstone arcaded structure built on an open square plan, fronted with a row of six Doric columns. The monument was restored under Italian rule in 1927-1928.
In Antiquity the acropolis was called Achaia polis, while after the Synoicism of Rhodes was known as Ochyroma (=Fortification). The modern name of Philerimos dates from Late Byzantine times and is contemporary with the vestiges of a defensive enclosure circling the top of the hill. The Hospitaller period (1309-1522) is marked by considerable building activity, which was responsible for the disappearance of much ancient construction. The medieval fort at the east end of the plateau is in fair condition, after an Italian restoration before World War II.
The medieval church of the Virgin on the site, dated to the early 14th c., replaced an earlier Byzantine church. In the 15th century two cross-vaulted chapels, now restored, were added to the church. The corbels of the north chapel bear the arms of grand master Aubusson, while a gothic niche in the south chapel sheltered the renowned icon of Our lady of Phileremos, the talisman of medieval Rhodes. The now heavily restored bell-tower, standing on top of the central apse of the Early Christian basilica, dates from the same period.
To the west of the ancient temple a Byzantine crypt was excavated out of the soft bedrock, containing a sacred fountain; it seems to have been part of a double church, which has almost completely disappeared. The fountain was later sealed off, and the attached cistern was turned into a chapel dedicated to St. George 'Chostos' which preserves interesting Western style14th c. frescoes.
The state of the site today owes a lot to large scale landscaping and restoration carried out by the Italians, who incorporated the medieval church into a monastery of the Franciscan Order. The restored church is now the Greek Ortodox rite Church of the Virgin.