In a landscape of magnificent spiritual beauty, and to the east of the sanctuary flows the sacred spring of Kastalia, gushing out from a rocky gorge in the base of Phlemboukos peak in the crevice formed by the twin cliffs (the two Phaedriades).

According to mythology this is the place where the god Apollo killed the serpent, Python. The water from the spring was directed towards the Kastalian Fountain (the Lower Kastalian), built about 600 B.C existing between the Temple of Apollo and the ancient gymnasium. A second fountain was constructed in the 1^st century B.C, approximately 50 metres closer to the spring.

It was from this spring water the Pythia, the priests and the temple workers, washed and purified themselves. The temple itself was also cleaned and purified using the sacred spring water.

Pilgrims and visitors, before consulting the oracle, were obliged to cleanse their souls by washing in the Kastalian Spring.