Kasern is a small district of the city of Salzburg in the very north. It is a rather funny extension into the municipalities of Hallwang and Bergheim, which was recently supplied with a local railway link (S-Bahn). Kasern has only a few hundred residents; the reasons why it is a part of Salzburg are historical. It was part of Bergheim, but was incorporated into Salzburg together with the Plainberg (the hill with Maria Plain) in 1939.
In 1950, the Plainberg was returned to Bergheim; in Kasern, however, there was a significant number of large industrial companies. In order to avoid a destruction of the "village features" of Bergheim, Kasern remained with Salzburg. The biggest company was a steel and bell factory, which closed in the 1990ies; the production halls now serve as an event location. There are fashion stores and other commercial zones in Kasern; the crane company Palfinger has its international headquarter in Kasern. There is a lot of traffic passing through Kasern, both on the road and railways.
Historically, Kasern has been an important administrative place, even though virtually unpopulated until well into the 19th century. On an extension of the Plainberg Hill (clearly visible from the train, by the way) was the Burg Radeck castle. It can be tracked back to the 13th century, belonged to civil servants of the Prince Archbishop, since 1273 to the Prince Archbishop himself. He made the castle a court. In the course of the peasant wars, Radeck Castle was burnt down in 1525, but rebuilt in subsequent years. After the secularisation of Salzburg, the castle fell into disrepair and was finally demolished. Today, you can only see the chapel of the castle.
German Wikipedia on Kasern
City of Salzburg, Official Website