The Monastery of Mega Spileo (Μονή Μεγάλου Σπηλαίου ) - formally the Monastery of the Dormition of the Theotokos (Ιερά Μονή Κοιμήσεως της Θεοτόκου) - is built on the western slopes of Mount Helmos at a height of more than 900 meters.

The monastery's architecture reminds of a fortress and it has eight floors. The cells of the monks are built around a cave. The monastery – one of the oldest of Greece - was originally built in 362 by two Thessalonian brothers, when a shepherd girl found an icon of the Virgin Mary inside a cave. The icon was made of wax and mastic and it is believed that Apostle Luke painted it. The icon survived a lot of fires and destruction and can still be admired in the monastery (PiP5).

In the year of 840 Mega Spileo Monastery was burnt down by a religious group. It was rebuilt in 1285 by the emperor Andronikos Paleologos, but suffered many destroying fires along its history. The monastery suffered another destruction in 1943 when the Nazi troops, after the massacre of Kalavryta, put the Monastery of Mega Spileo on fire and killed its twelve monks and its staff, either shooting at them or throwing them from the rocks. Some managed to survive, hidden in close caves after having taken some valuable icons with them.

Nowadays the Monastery of Mega Spileo is an imposing construction with a 17th-century church that hosts remarkable frescoes, mosaic floors and a bronze door with relief decoration. The Monastery also hosts a museum that displays carved wooden crosses, antique manuscripts, and Holy Gospels.