Venice was founded on marshy islands in the fifth century and grew into a major trading republic by the Middle Ages. Its fleet controlled routes across the eastern Mediterranean, and its merchants financed trade in spices, silk, and glass. The city's political system was deliberately complex, with elected doges and councils designed to prevent any single family from dominating. The canals and palaces that survive today draw millions of visitors each year, although rising water levels now threaten parts of the historic center.