September 11th is also the date when the Turks were turned back from Vienna by Jan Sobieski in 1683. The Battle of Vienna is one of the big turning points of history, when Central Europe began to flourish because effort could be devoted to things other than preparing for and waging war. All the beauty of Vienna dates after this, because everything outside the walls was regularly destroyed.
If I had know this when we was there, I would have looked for them. I don't know whether they can be seen, but that would have been fun to see when we were there.
I was probably too full of Wienerschnitzel to go looking at the time though.