which landmark built on the site of a former city gate that marked the start of the road from Berlin to the town of Brandenburg an der Havel, is Frederick William II of Prussia responsible for?
Frederick William II of Prussia:  However, he was a patron of the arts and responsible for the construction of some notable buildings, among them the Brandenburg Gate in Berlin. Brandenburg Gate:  One of the best-known landmarks of Germany, it was built on the site of a former city gate that marked the start of the road from Berlin to the town of Brandenburg an der Havel, which used to be capital of the Margraviate of Brandenburg.
Brandenburg Gate