I’ve been travelling by train across Central Europe this hot summer and, as often happens with Americans, I’ve been reminded of the sheer density of human history in older corners of the world. On Sunday morning, for instance, I spent a few hours at Hrad Devín, or Devín Castle, a stone ruin a dozen kilometres upriver from the center of the (low-key and utterly charming) Slovakian city of Bratislava, on the Austrian border, at the spot where the bluish-green Danube meets the olive Morava, flowing in from the mountains of the Czech-Polish border. It’s such a clearly strategic spot that it’s no wonder people have been settling here for millennia. There are excavations of an old Celtic community from the first century B.C., ultimately replaced by a Germanic population that established links w...