First mentioned in 1147, the fortified town of Moscow became an important center during and after the Mongol rule in Russia. With its central location near the Volga River and its role as sole tax collector for the Mongol Golden Horde, Moscow became the most important city in Russia and the main seat of the Russian tsars from Ivan the Terrible until Peter the Great. Today, with a population of over ten million, the city is the largest in Europe and recently ranked the most expensive in the world. The American Center in Moscow opened in June 1993.