The Frankish missionary Rupert (Hroudbert) came to Salzburg around 700 and founded St. Peter's Church and a monastery on the soil of what was once the Roman Juvavum. It remains the oldest continuously active monastery on German soil.

Salzburg was designated an archbishopric under Archbishop Arno, a friend of the Emperor Charlemagne. The church and its buildings were destroyed by fire in May 1127; from 1130 to 1143 Abbot Balderich had a three-naved Romanesque basilica built. The church and the church district were subject to numerous adaptations in the years to follow. One of Salzburg's oldest Gothic buildings was built in 1319, the St. Veits Chapel. The magnificent Romanesque portal was built around 1240. The transformation to the Rokoko style took place in the 18th century under the energetic and art-loving abbot, Beda Seeauer. During the first half of the 20th century Archabbot Petrus Klotz founded the Collegium Benedictinum with fa?ade frescoes done by Anton Faistauer.

The interior of the church still gives a clear impression of the Romanesque style. The Rokoko decoration of the church is reflected in the large number of altars. Several of the altar paintings are the work of Martin Johann Schmidt, also called "Kremser-Schmidt", one of the most famous devotive and altar painters of the 18th century. Two gigantic Renaissance bronze candlesticks dating back to 1609 were donated by Archbishop Wolf Dietrich. In one of the chapels located on the right side aisle are the memorial of Johann Michael Haydn, a marble plaque in memory of Mozart's sister Nannerl and the tomb of Hans Werner von Raitenau, Wolf Dietrich's father.

Prominent personalities belonged to the monastic community of St. Peter. The scholarship and the fervor of these medieval monks are reflected in a multitude of works. A masterpiece of the prevailing architecture was the construction of the Alm Canal, which involved digging a tunnel through the Mönchsberg. Many of the monks were employed as teachers after the founding of the University in 1622; music and theater were avidly fostered in St. Peter's. In 1769 thirteen-year-old Mozart composed the "Dominicus Mass" for Abbot Hagenauer and in 1783 personally directed his famous Mass in C-minor. In 1782 Johann Michael Haydn composed the "Rupert Mass" for St. Peter's.

Source from :http://www.salzburg-visitorscenter.com/information/stpeter.htm