St. John's Church was built between the 12th and 16th centuries, though there must have been churches preceding it. It was heavily damaged during World War II, but it has been restored. The oldest part of the building is the Romanesque tower. The tower dates from the early 13th century.
The choir, in part likewise from the 13th century, is a mixture of Romanesque and Gothic elements, and features two side chapels. To the left of the choir is the vestry, above which there is a room that must once have served as a chapel.
The 14th-century nave is now quite low, as it is covered by a modern ceiling inspired by that of the vestry.
The chapel to the south of the nave dates from the 15th and 16th centuries. Its two bays have Gothic vaults. The vaults of the northern aisle, which an inscription dates 1513, are flamboyant Gothic.
References:The Clementinum is a historic complex of buildings in Prague. Until recently the complex hosted the National, University and Technical libraries, the City Library also being located nearby on Mariánské Náměstí. The Technical library and the Municipal library have moved to the Prague National Technical Library at Technická 6 since 2009. It is currently in use as the National Library of the Czech Republic.
Its history dates from the existence of a chapel dedicated to Saint Clement in the 11th century. A Dominican monastery was founded in the medieval period, which was transformed in 1556 to a Jesuit college. In 1622 the Jesuits transferred the library of Charles University to the Klementinum, and the college was merged with the University in 1654. The Jesuits remained until 1773, when the Klementinum was established as an observatory, library, and university by the Empress Maria Theresa of Austria.