Amsterdam is situated at the mouth and on the south side of the IJ, an inland arm of the former Zuiderzee, now IJsselmeer, and connected by canal with the North Sea.
It is divided by the canalized Amstel River into two main sections.The medieval town lies on either side of the Amstel at the city’s centre, enclosed by the semicircular Singel (ditch or moat). Outside the Singel are the three main canals dating from the 17th century: the Herengracht (Heren Canal), Keizersgracht, and Prinsengracht. Within this area smaller canals run north and south. One tower (the Schreierstoren) of the old fortifications still stands.