For centuries, Vienna was the “gateway” to the west for Muslim invaders. Two times- once in 1529, and the other in 1683, Muslim armies reached the walls of Vienna but were not able to pass them.
History is repeating itself again, as following this massive “migration” of Muslims to Europe, the once-liberal Austrians are sick of allowing Muslim invaders to pour over their borders and in response they are building a massive wall to keep them out following similar plans from Eastern European nations:
http://shoebat.com/2016/07/22/austria-says-no-more-muslims-finalizes-plans-to-build-a-massive-wall-on-the-border-to-keep-the-muslims-out/