The first site one comes across as one descends from Castel Gandolfo towards the lake is the Doric Nymphaeum, located in a higher position compared to the other monuments, which are instead lapped by the waters of the Albano lake.
Originally part of the extensive residence of Domitian that ranged all around the lake, now the Nymphaeum appears isolated and perfectly preserved in all its beauty and magnificence. Built inside the lake crater, it faces the monte Cavo, the ancient mons Albanus (a political and religious seat of the Latin League).
Discovered by chance at the beginning of the 18th C., the Nymphauem was generally attributed to the Age of Domitian (1st C. A.D.) but many scholars believe that owing to its rigorous architecture and its monumental quality it could be identified as one of the “sacella” (sacrarium), described by Cicero and built by Clodius on the ruins of the ancient Alba Longa which, in all probability, stood on the area of ground that now hosts Castel Gandolfo. It is 11 x 6 mt in size, with the barrel vaulted ceiling reaching a maximum height of 8 metres. The Nymphaeum is faced on the inside with opus reticolatum panelling.
The variety of decorative styles ranges from the Doric cornice to the capitols and the consoles of the pilasters which are instead Ionic. The series of niches that run along the sides of the space, including the two on the far wall beneath a broken tympanum (or gable), lead us to believe that they contained statues. Beneath the tympanum there used to be an actual motor which powered the amazing water works achieved with larger and smaller waterfalls and channels that were fed with the water from one of the many aqueducts that ran through the Albano crater and by a series of cisterns and hydraulic ducts installed behind the central back wall. In medieval times, the area in front of the Nymphaeum was probably fortified, but there is very little archaeological evidence to help us understand this particular phase.