St. John's Church was built in the 13th century, probably between 1234-1297. It was originally home of the Dominican monks, but over the centuries fell into the possession of the Lutherans. It has also served as an arsenal for the city.
The most notable features of this unheralded church are the impressive 15th century sculptures of St. Peter and St. Paul which adorn the the 18th century altar. According the legend two monks who were bricked to the southern wall during the construction. They spent all their life long and were fed trough a window from the outside.
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.