Antiquity
The ancient polis of Kalaureia was home to an asylum dedicated to Poseidon, the ruins of which are still accessible on a hilltop close to the town. This asylum may have been linked to the sanctuaries at Geraistos and Tainaros. Ancient historians claimed that Poros was home to an Amphictyony in the Archaic period, a league of the poleis Poros, Athens, Prassiai, Aegina, Epidaurus, Hermione, Troizen, Nauplion and Orchomenus. However, there is no evidence for this, and modern scholars believe the Amphictyony may have been a Hellenistic invention. An enormous feast was found dating to the Hellenistic period in the ruins of the Kalaureia asylum, along with a plaque celebrating the "revival" of the Kalaureian League.
Byzantine/Ottoman Era
Further information: Byzantine Greece and Ottoman Greece
In Byzantine times, Poros and other islands were often raided by pirates. During the Ottoman rule Poros remained independent, and helped neighbouring islands after the start of the Greek War of Independence.
Russian Naval Base
With the Treaty of Kuchuk Kainarji, Russia secured free shipping for its navy, war and merchant alike, throughout the waters of the Ottoman Empire. As Russian naval activity grew, need arose for a supply station, and land was acquired at the edge of Poros town. Extensive materiel, coal, and food storage facilities were built, as well as a hardtack baking factory. After Greek independence, Governor Capodistrias requisitioned the facilities for use of the Greek war navy, and offered the Russians an alternative location in a nearby cove. The new facilities were far larger, and were used by Russian ships throughout the 19th century. The number of Russian residents of Poros increased and even a Russian school was established. Then as Russian naval activity declined, so did the base and by the early 1900s only a single Russian watchman was left guarding it. It was then granted to the Greek
