The first documents testifying to the existence of a castle in Castro date back to the 13th century: a document of 1282 described it as having a strategic importance to protect the kingdom. It is quite sure that the castle was built on the ruins of a former Byzantine building, which protected the fortified Roman village. In 1480 it was destroyed and then rebuilt and strengthened  in 1572 by the Spanish viceroy.
On the north-western part, stand the thick walls of the Bastion of the buttress, with a single central room and fortified battlements. The Torre Cavaliera, the highest and most imposing one, is a three-story tower with three rooms, covered with barrel vault. The Torre Circolare, dating back to the Aragonese period, has three rooms with small windows and merlons with machicolation.
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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.