Corfu or Kerkyra is a Greek island in the Ionian Sea. It is the second largest of the Ionian Islands, and, including its small satellite islands, forms the northwesternmost part of Greece. The island is part of the Corfu regional unit, and is administered as a single municipality, which also includes the smaller islands of Ereikoussa, Mathraki and Othonoi. The municipality has an area of 610,9 km2, the island proper 592,8 km2. The principal city of the island and seat of the municipality (pop. 32,095) is also named Corfu. Corfu is home to the Ionian University. The island is bound up with the history of Greece from the beginnings of Greek mythology. Its history is full of battles and conquests. Ancient Korkyra took part in the Battle of Sybota which was a catalyst for the Peloponnesian War, and, according to Thucydides, the largest naval battle between Greek city states until that time. Thucydides also reports that Korkyra was one of the three great naval powers of fifth century BC Greece, along with Athens and Corinth. Medieval castles punctuating strategic locations across the island are a legacy of struggles in the Middle Ages against invasions by pirates and the Ottomans. Two of these castles enclose its capital, which is the only city in Greece to be surrounded in such a way. As a result, Corfu's capital has been officially declared a Kastropolis ("castle city") by the Greek government. From medieval times and into the 17th century, the island, having successfully repulsed the Ottomans during several sieges, was recognised as a bulwark of the European States against the Ottoman Empire and became one of the most fortified places in Europe. The fortifications of the island were used by the Venetians to defend against Ottoman intrusion into the Adriatic. Corfu eventually fell under British rule following the Napoleonic Wars. Corfu was eventually ceded by the British Empire along with the remaining islands of the United States of the Ionian Islands, and unification with modern Greece was concluded in 1864 under the Treaty of London. In 2007, the city's old quarter was added to the UNESCO World Heritage List, following a recommendation by ICOMOS. Corfu is a very popular tourist destination. The island was the location of the 1994 European Union summit.