In the 12th and 13th centuries, this enormous complex of buildings was the Prague headquarters of the Inquisition. When the Jesuits moved in during the 16th century, kicking out the Dominicans, they retained some of the Holy Office's less savoury practices, including the forcible baptism of the city's Jews. They also replaced the medieval Church of St Clement with a much grander design of their own (rebuilt in 1711-15 and now used by the Greek Catholic Church), while also gradually constructing the building of today. It's arranged around five courtyards, and several streets and 30 houses were demolished during its construction.
The Jesuits' grandest work was the Kostel sv. Salvátora (Church of St Saviour), which has an opulent but grimy façade that was designed to reawaken the joys of Catholicism in the Protestant populace. Built between 1578 and 1653, it was the most important Jesuit church in Bohemia.
However, it's not the only superb building here. The Jesuits' main tool was education, and their library is a masterpiece. Completed in 1727, it has a magnificent trompe l'oeil ceiling showing the three levels of knowledge, with the Dome of Wisdom occupying the central space. However, the ceiling started crumbling; to prevent the whole structure from collapsing, the Zrcadlová kaple (Chapel of Mirrors) was built next door in 1725 to bolster the walls. The chapel interior, decorated with fake pink marble and the original mirrors, is lovely, and there are two 18th-century organs.
At the centre of the complex is the Astronomical Tower, where Kepler came to stargaze. It was used up until the 1920s for calculating high noon: when the sun crossed a line on the wall behind a small aperture at the top, the castle was signalled and a cannon fired.