Built at the beginning of the 2nd century AD, the Selley Theatre could accommodate around 12,000 spectators, making it one of the largest buildings of its kind. It was modified in the Gallo-Roman age. Both the orchestra and the cavea (tiered seating) are more than semicircular, the rather modestly sized stage building was built onto the exterior of the linear façade wall, and the actual stage consists of a simple wooden platform which reaches into theorchestra.
In the latter part of the 3rd century i twas finally turned into a fortified retreat enclosed by a defensive ditch and was used as such until the mid-4th century.
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.