The Castalian spring was the sacred source of Delphi, and its water played an important role in the cult and procedure of the temple and of the oracle. This is where Pythia, the priests and the temple staff washed, and where the water used to clean the temple came from. The theopropoi - those wishing to consult the oracle - were also obliged to wash here in order to purify themselves.

The Castalian spring is located at the foot of the rocky crag Phleboukos (ancient Hyampeia), inside the ravine separating the two Phaedriades. Its waters form a stream, the so-called Arkoudorema, which runs into the Pleistos valley where, according to a myth, the Python had its lair. Water was also channeled to the homonymous fountain situated between the temenos of Apollo and the ancient gymnasium.

The Castalian fountain was originally built around 600-590 BC near the ancient route, and was refurbished several times since. Still visible today on the modern roadside, this so-called Lower Castalia is a rectangular structure of 8.20 by 6.64 metres enclosing a rectangular stone-built basin with a system of pipes and spouts. In front of the basin is a paved terrace with stone benches, reached by a small staircase. The water was channeled from the spring by a rock-cut subterranean pipe.

The later Castalian fountain, which dates to the first century BC and is located approximately fifty metres uphill, closer to the spring, is the one described by Pausanias. The fountain was built inside a rock-cut hollow eleven metres long by twelve and a half metres high, supplied with niches for the votive offerings to the nymph Castalia. The largest of these niches became a chapel of Saint John the Baptist in the Post-Byzantine period. Under the niches is an elongated rock-cut basin, ten metres long by half a metre wide, which was covered and had an opening for cleaning at one end. A closed pipe supplied the water. On the basin's fa?ade were seven bronze spouts separated by seven engaged rock-cut columns. In front of the basin was a paved terraced with stone benches, accessed from above by eight steps.

The later fountain was excavated by S. Dragatsis and E. Kastorchis in 1878, and the Lower Castalia by A. Orlandos in 1960. Both monuments have been conserved and restored.