Sitting right next to the Charles Bridge on the eastern flank of the Vltava River, the Klementinum (Clementinum) is a vast complex of buildings covering fives acres (two hectares). It started life in 1232 as a place of learning for medieval scholars before becoming a Jesuit monastery in 1556. For the next two centuries, it slowly expanded into one of the biggest learning centers in Europe, featuring an astronomical observatory, baroque libraries smothered with frescoes, and ornate apartments full of marble and gilt. It then served as a university with study rooms, lecture halls, and dormitories until around 1930 when the Klementinum became the Czech National Library—20,000 books and manuscripts are housed in the Baroque Library Hall, which is a masterpiece of 17th-century frescoes by Jan Hiebl.
Equally startling is the Mirror Chapel. Designed in 1724 and also rich in flamboyant baroque flourishes, the chapel serves as a venue for nightly classical concerts and features an organ played by Mozart. The Klementinum’s Astronomical Tower offers some of the best views of Prague and the river (it’s an arduous, 172-step climb up, however), while the site's two churches are widely considered to be the best examples of baroque architecture in the city.
The Klementinum is open daily from 10am to 4:30pm for guided tours only. The site is easily reached on foot from Prague's Old Town.