Leaving aside the remnants of the first Romanesque church of the Virgin Mary, there is the St. George Basilica, which is the oldest religious building of the castle. The church was established around the year 920 by Vratislav I. It served as a burial ground for the remains of the dynasty Přemyslovi, for example, the remains of the St. Ludmila were transported and stored here after she was murdered. The site is also significant because it was the first monastery in the country (Benedictine nunnery). From an architectural point of view, the basilica is an exceptionally well-preserved Romanesque building. After the devastating fire, its facade was rebuilt in the Baroque style and it was designed by Francesco Caratti. The church is a three-aisled basilica with two towers, and inside there is a square chancel with an apse and crypt. The Chapel of St. Ludmila dates from the 12th century. In the 18th century, the Chapel of St. Jan Nepomocký was added to the western facade of the basilica by the architect Franz Maximilian Kaňka.
The significant Benedictine monastery was like many other monasteries that were abolished during the reign of Joseph II. In the 60s of the 20th century it was renovated for the needs of the National Gallery.