Sant'Orso had originally a single hall, delimited by a semicircular apse. It was entirely rebuilt during the 9th century, during the Carolingian age. Later, bishop Anselm of Aosta further renovated the church, introducing a basilica plan with three naves with wooden trusses. These were replaced by Gothic cross vaults in the 15th century.
The church has a nave and two aisles divided by quadrangular pillars.
The vault was rebuilt in the 15th century. Fragments of a Romanesque series of paintings are preserved in good condition in the space between the current vault and the original ceiling. These portray scenes from the New Testament as well as a martyrdom. Stylistically they resemble the bright colours and strongly marked outlines of some of the frescoes at the Galliano Basilica near Cantù. In the right aisles is a chapel houseing the altar of St. Sebastian, also with frescoes (15th century).
The cloister has historiated capitals depicting the life of Ursus. 37 of the 42 original capital remains: they were originally in white marble, though now they mostly appear in dark gray color aftery they were washed with ash paint.
The quadrangular-plan bell tower, dating to 989, has kept some the lower 15 metres of the original medieval structure. The present structure, in Romanesque style, dates to the 12th century, and has a total height of 44 metres.
The church is home to numerous missals and reliquaries, including the relics of Ursus, which rest in the crypt. It also holds the relics of Saint Gratus of Aosta.
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.