Boppard, Germany: Roemer-Kastell Bodobrica

The Romans showed up here and decided that the city's location along the Rhine was important enough for some sort of defensive structure. So, they build a fort in the fourth century.

The Romans are gone now, but they were such good builders that signs of their time here are still around. (This is true almost everywhere, naturally.)  It's fun to stroll through the part-like setting and ponder just how old the walls and gate are. It's worth a 10-minute visit if you are in the area.