St. Peter's Church have been probably built in two phases during the 12th century.The eastern part with chancel, transept and central tower were erected first during the late 1100's, while the nave and the present west tower were added later.
According the tradition the church was used as a bishop’s cathedral until 1130 when the bishop's seat was moved to Gamla Uppsala. The another legend believes the church has been a royal church. After a fire in the 1600s, the church has stood as a ruin.
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.