Third Fortification of Salzburg:
In the course of the Thirty Years′ War, Prince Archbishop Paris Lodron grasped that Salzburg was "accessible like a village". He vamped up taxes by some 30 percent, thereby causing a steep increase in poverty and a recession; however, he managed to raise vast sums of money this way, which he invested into the "Third Fortification". Most of the walls and gates that you can still see today date back to this period. The construction of the bastions around the Fortress Hohensalzburg saw their prime, but also the Nonnberg (Schanzlgasse) and the northern parts of the Mönchsberg (Müllner Schanze) were fortified. The architects in charge were imperial engineers, the court builder Santino Solari was in charge with supervising the construction work.
New gates were the Erentrudistor, the Griestor (or Wassertor), the Siegmundstor, the Ruperttor with the Rupert Bastion, the Heinrich Bastion, the Virgil Bastion and the Vitalis Bastion. New gates were built for the Kapuzinerberg (Felixtor, Franziskustor), alongside with massive walls; the cliffs of the Kapuzinerberg were made steeper; the Trompeterschlössl was replaced by the Franziskischlössl as the main stronghold on the Kapuzinerberg. Many of the gates that originated in the Second Fortification were extended and modernised. In 1641, the first army barracks on German soil are built at today′s Griesgasse, the so-called Türnitz.
By the end of this building campaign, Salzburg was one of the most heavily fortified cities in Europe. As a result (and for other reasons as well, but we shall simplify a little bit here), Salzburg never got involved in combat in the course of the Thirty Years′ War, whereas neighbouring Bavaria lost about a third of its population. Soon after the war ended, the significance of the fortifications started to decline gradually with the development of ever more sophisticated weaponry. Few modernisations took place (such as the construction of the outer Linzertor in 1704), but simultaneously, the walls hindered the growing population to expand the city.
1832 marks a turning point: The Äußeres Steintor was demolished, the first in a series of demolitions. The walls and gates often served as quarries or were simply removed to open construction space in the 19th century. In 1860, the construction ban for the areas near the bastions was formally lifted. In 1862, the Salzach River gets trained and large blocks of stone are needed. This is the death sentence for several gates that get demolished until 1866 (Lederertor, Virgiltor, Äußeres Linzertor).
Emperor Franz Joseph I gives most sections of the walls as a gift to the city of Salzburg in 1866; the bastions outside of Linzergasse are demolished, a new neighbourhood is developed instead: The Neustadt. A few more demolitions follow, such as the Kajetanertor in 1873, the guard houses at the Neutor in Riedenburg in 1874 or the Inneres Linzertor in 1894. The Alte Frohnburg is demolished in the early 20th century to create space for the Landesgerichtsgebäude. And in the course of World War II, bomb damages destroy some parts of the walls and fortification, most famously the Hexenturm.
City Walls on the SalzburgWiki (German)
German Wikipedia on Paris Lodron