Moscow is situated on the Moskva River in the Central Federal District of European Russia. In the course of its history the city has served as the capital of a progression of states, from the medieval Grand Duchy of Moscow and the subsequent Tsardom of Russia to the Soviet Union. Moscow is the site of the Moscow Kremlin, a medieval city-fortress that is today the residence of the Russian president. The Moscow Kremlin is also one of several World Heritage Sites in the city. Both chambers of the Russian parliament (the State Duma and the Federation Council) also sit in Moscow. The city is served by an extensive transit network, which includes four international airports, nine railway terminals, and one of the deepest underground metro systems in the world, the Moscow Metro, third to Tokyo and Seoul in terms of passenger numbers. It is recognized as one of the city's landmarks due to the rich and varied architecture of its 188See More