Lake Orestiada and the city of Kastoria are indissolubly tied for many centuries.
The lake is estimated to have been formed 10,000,000 years ago (in the Miocene), and its present form is a remnant of an older extensive lake with an area of 164 square kilometers and a depth of more than 50 meters.
It is considered to be a lake of karstic, tectonic origin with a total area of 28 square kilometers, average depth of water 4,40m and maximum depth 8,50m.
It has been designated as a site of special beauty by the Ministry of Culture (1974) and has been included in the European Network "NATURA 2000" registered under the code GR1320001 as well as in the list of important areas for the birds of Greece and Europe (SRA).
The lake and the surrounding area are a very important natural ecosystem with diverse and rare habitats that support a large biodiversity including many rare and endangered species.
Besides the very large number of birds that host it, it is the only natural lake in Greece, where there are still significant stretches of riparian forests of hydrophilic trees, which today constitute one of the rare habitats of the European area.