Question: What is the name of the town square that is on the border of the first and fourth districts of Vienna and one of the most frequented and best connected transportation hubs in Vienna, the location of Karlskirche which is  widely considered the most outstanding baroque church in Vienna?
Sentence (from "Johann Bernhard Fischer von Erlach"): His influential book "A Plan of Civil and Historical Architecture" (1721) was one of the first and most popular comparative studies of world architecture.
Relevant evidence: No