Sacellum: Chapel of the Old University
The Sacellum is a chapel that was built in 1618 for the campus of the Old University of Salzburg. It is one of the oldest structures of the old campus. Today, it is used by catholic student organisations, but accessible only at very limited times. You can, however, see much of the chapel from the outside. Note that it is a "Hauskapelle" and incorporated into a secular building; therefore, you might easily miss it and walk by unintentionally. You can find the Sacellum opposite to the Festspielhaus (Festival Hall) on the corner near the Pferdeschwemme.
The Sacellum has a vaulted ceiling over a rectangular nave. On the ceiling, there are paintings that depict the 15 secrets of the rosary (apparently giving them away, so strictly speaking, the 15 former secrets of the rosary). These paintings were quoted in paintings that decorated the Große Aula. The original Sacellum was extended in 1663.
The main altar of the Sacellum is made of marble and was built by Wolfgang Hagenauer between 1766 and 1768. Two side-altars were made around 1750. Another gem that the Sacellum contains are relief decorations made of lead; they depict scenes from the life of St. Borromaeo (the patron saint of the chapel) and St. Benedict (the university was "run" by the Benedictine monks of St. Peter). There is a sacristy and a small side-chapel to the Sacellum. The building is used for services of catholic organisations that are associated with Salzburg University and for lectures, discussions and cultural events.
Hidden Treasures of Salzburg
German Wikipedia on the Old University and Sacellum