Amsterdam has more than 100 kilometres (60 miles) of canals, most of which are navigable by boat. The three main canals are Prinsengracht, Herengracht, and Keizersgracht. In the Middle Ages, Amsterdam was surrounded by a moat, called the Singel, which now forms the innermost ring in the city, and makes the city centre a horseshoe shape. The city is also served by a seaport. It has been compared with Venice, due to its division into about 90 islands, which are linked by more than 1,200 bridges.
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