The remains you see here were part of the Basilica of St. Euphemius built in 1031 by Litigerius, bishop of Como. It was demolished by the city of Como in 1169 (historical sources document the deed of foundation). Archaeological excavation has revealed the existence of an earlier religious building, erected during the Early Christian period or the Early Middle Ages. The earliest ruins date back to Roman times.
The Romanesque building was a large three-aisled basilica (22×62 m); the aisles, separated by octagonal masonry columns, terminated in three semicircular apses facing east. A central staircase of 9 steps allowed visitors to ascend to the raised presbytery while in the wings two smaller staircases led down to the crypt. Given the composition of the terrain (for the most part bedrock), the right apse was placed at the same level as the aisles while the left apse was built on a somewhat higher level.
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.