The extensive grounds of the Clementinum, one of the largest building complexes in Europe, were built from the mid-16th century to the mid-18th century, originally as a Jesuit dormitory. In its Astronomical Tower, meteorological measurements have been collected since 1775. The most beautiful hall of the complex is the Baroque Library with beautiful frescoes and historically valuable globes. The Mirror Chapel with its richly designed interior and unique installation of mirrors is a place where classical music concerts take place regularly.Clementinum is the largest complex of buildings except for the Prague Castle (Pražský hrad). It is the first college in Prague where the Jesuits came in 1556. Ferdinand I called them up to Prague in order to help him in his Counter-Reformation effort. After arrival, they settled in a former Dominican monastery of St. Clement in the Old Town (Staré Město) next to the Charles Bridge (Karlův most). This small church later gave a name to the entire complex – Clementinum. In the period of recatholisation after the defeat of Bohemian Estates in 1620, there was a great building boom which continued till the middle of the 18th century. The Jesuit college complex of buildings was built on spacious grounds where previously stood 32 houses, seven courts, three churches, a monastery and several gardens.