We do not know much about the following periods, except that the island joined the Athenian Alliance, after the Persian wars , like most islands in the Cyclades,under the name of Keria. During the Middle Ages, Keros, like the whole cluster of the Little Eastern Cyclades, was a base and a refuge for pirates,meaning that the economy and life of its inhabitants were determined by the activity of piracy.
Today, the only signs of life on the island are some types of accommodation in the area ``Konakia`` in the middle of the north coast, for farmers who visit the island seasonally, as well as small chapels (Konakia, Panagitsa). There is a very rough road network of barely passable and narrow dirt roads that crosses the center of the island and is used only by farmers. An extensive fortified settlement of the same period has also been located in Daskalio.
Due to its cultural richness, Keros has been repeatedly looted in the past by antiquity traffickers. Finally,this is where the so-called ``Treasure of Keros``comes from.It used to be part of the Erlenmager collection, while today, most of it is exhibited in the Museum of Cycladic Art.
Organised official excavations were carried out by the Archaeological Service in 1963 and 1967. Finds from Keros are exhibited in the Museum of Naxos. Over the first 10 years starting in 2000, a number of archaeological finds came to light after excavations carried out by the team of archaeologist Professor Colin Renfrew.
Archaeological exhibitions with artifacts found in Keros are hosted in the museum room situated on the main beach of our island.
A unique experience for every visitor in Koufonisi are the full moon nights when the view of Keros resembles the outline of a lying couple.The pregnant woman is lying with her figure perfectly outlined clear, head to head with the man, whose bust is clearly visible.
Keros is uninhabited today and not accessible for vacationers, due to the archaeological excavations that are carried out there from time to time. According to the locals, the energy that this island emits creates this pleasant atmosphere and serenity characteristic of Koufonisia.