The origins of this massive complex—now part of the university—date back to the 12th and 13th centuries, but it's best known as the stronghold of the Jesuits, who occupied it for more than 200 years beginning in the early 1600s. Though many buildings are closed to the public, it's well worth a visit. The Jesuits built a resplendent Baroque Library, displaying fabulous ceiling murals that portray the three levels of knowledge, with the "Dome of Wisdom" as a centerpiece.
Next door, the Mirror Chapel is a symphony of reflective surfaces, with acoustics to match. Mozart played here, and the space still hosts chamber music concerts. The Astronomical Tower in the middle of the complex was used by Johannes Kepler, and afterward functioned as the "Prague Meridian," where the time was set each day. At high noon a timekeeper would appear on the balcony and wave a flag that could be seen from the castle, where a cannon was fired to mark the hour.