The Barrage Vauban. We discovered the best way to get there and experience it is enter the south end, walk through the inside. There are some statues that have been removed from a church, including several gargoyles, so it is really a must see, lol. At the north end climb the stairs and walk back to the south end taking in the views of the area.
Besides all the wonderful views and sights in the old city section of Strasbourg, it also very small and one can walk from one side of it to the other in only twenty minutes. Of course will all the things to stop and look at it takes far longer. One guide book said you could see all of Strasbourg in three hours. It was a book for tourists not travelers like us who like to stop and savor where we are. The unique blend of French and German culture is something to be experienced, not just seen in passing.