St. Nicholas Church is a large, cruciform town church set on Place Dalton at the heart of the lower town which clusters below the walls of the citadel. Due to the destruction of the French Revolution, WWI and WWII it is the most significant surviving medieval building in the town.
The church is first mentioned in 1208 as a foundation of the Abbey of Notre-Dame within the citadel, a relationship which existed into the 16th century. About this time, the silting up of one of the channels caused the centre of the town to move southwards, and St Nicholas took over from St Peter as the main town church.
The vaulted roofs in the nave and transepts were installed in the 17th century, but their weight caused the walls to crack, and as part of the rebuilding the nave was widened and lengthened, a response to the doubling in population of the lower town that century.
All the glass was destroyed during the liberation, and has so far been replaced in the lower stories with some splendid 1980s abstract designs.
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.