Corfu 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 edge of the northwestern frontier of Greece. The island's history is laden with battles and conquests. The island is bound with the history of Greece from the beginning of Greek mythology. Its Greek name, Kerkyra or Korkyra, is related to two powerful water symbols: Poseidon, god of the sea, and Asopos, an important Greek mainland river. The legacy of these struggles is visible in the form of castles punctuating strategic locations across the island. Two of these castles enclose its capital, which is the only city in Greece to be surrounded in such a way. In 2007, the city's old city was designated for the UNESCO World Heritage List.