The oldest Christian religious structure on the Feneos plateau is the Palaiomonastiro, or more widely known as the Chapel of Saint Phanourios, located in the middle of Lake Doxa. The small church served as the katholikon (main church) of the first monastery built in the Feneos plain in the 13th century AD and was originally dedicated to Saint George.

In the 17th century (1693), when the Feneos Lake flooded the entire plain, the monks were forced to abandon the original monastery and build a new one at a higher elevation, beyond the reach of the waters of the Olvios and Doxa rivers. However, the katholikon of the old monastery remained intact through the centuries, and in 1991, it was dedicated to Saint Phanourios to ensure it would not remain unused.

The creation of the artificial lake, with the dam built in the former bed of the Doxa River, combined with the preservation of the small Byzantine chapel, has created a unique landscape. A narrow strip of land now forms a small island, on which the chapel still stands, earning it the nickname "the Mouse Island of Corinthia." According to the 6th Ephorate of Byzantine Antiquities, it is classified as a Byzantine monument and celebrates its feast day annually on August 27.

Architecturally, the chapel itself is unremarkable, but in combination with the unparalleled natural beauty of its surroundings, reflected in the lake's tranquil waters, it creates a unique and idyllic scene that attracts thousands of visitors each year. Over the past three decades, it has become the defining symbol of the greater Feneos area.