Frigid Fracas by Mack Reynolds

Budapest, basically, had changed little over half a millennium.

The Danube, seldom blue except when seen through the eyes of a twosome between whom spark has recently been struck, still wandered its way dividing the old, old town of Pest from the still older town of Buda. Where the stream widens there is room for the one hundred and twelve acres of Margitsziget, or Margaret Island to the West-world. Down through the ages, through Celts and Romans, Slavs and Hungs, Turks and Magyars, none have been so gross as to use Margitsziget for other than a park.