The Michaelitor was one of the most important gates and entrance points to the city of Salzburg. It was part of the powerful fortifications that Prince Archbishop Paris Lodron built in the 17th century, when he felt threatened by the 30-Years′-War. The Michaelitor secured the Rudolfskai area, and was part of the might walls along the River Salzach. Today, the site of the gate is the link between the Mozartplatz Square and the Mozartsteg Bridge.
There is only one section of the Michaelitor that was preserved: The so-called Zeugwartstöckl, the residential tower for the gate keepers, which was built in 1628. it is a noteworthy building with a nice sgraffito sun dial on the wall. It is the oldest part of the fortifications; the Michaelitor itself was developed later on to create residential space for soldiers. These residences were gradually built between 1720 and 1830.
After Salzburg had recovered from the trouble and depression of the Napoleonic Wars, its population increased rather dramatically. The 17th century walls proved to pose a problem on traffic and transportation, as well as building activities. Several gates and bastions were demolished, among them the Michaelitor. It was removed in 1866, the Zeugwartstöckl, however, remained at its site and is preserved in excellent condition.
Michaelitor on SalzburgWiki (German, but with pictures)