Prague Castle is the largest ancient castle complex in the world by area, seventy thousand square meters above the Vltava. The first fortifications were built in the late ninth century by the Přemyslid dynasty. Bohemian kings expanded it, Holy Roman Emperors extended it, Habsburgs added to it, and the modern Czech presidency uses it. Walking from the south gate through three successive courtyards to St Vitus Cathedral's apse is a one-kilometre walk that crosses a thousand years of architectural memory. Few sites in Europe compress this much continuous occupation into one footprint.

Two periods deserve specific notice. The first is the reign of Charles IV in the fourteenth century, when the Bohemian crown was also the Holy Roman imperial crown. Charles brought the French master mason Matthias of Arras to Prague in 1344 to begin St Vitus Cathedral. He founded Charles University, the first university in Central Europe, in 1348. He built the Charles Bridge across the Vltava, the bridge in the foreground of the view above. For a generation, Prague was the most ambitious capital in Europe.

The second is the reign of Rudolf II at the end of the sixteenth century. Rudolf moved the imperial court from Vienna to Prague Castle in 1583 and turned the upper rooms into the strangest court in Europe: a working laboratory for alchemy, astronomy, art, and esoteric scholarship. Tycho Brahe was his imperial mathematician. Johannes Kepler succeeded him. Edward Kelley and John Dee passed through. Arcimboldo painted his composite-fruit portraits there. The court's intellectual seriousness and its credulity about magic existed in the same rooms at the same time, on the same payroll. The Habsburgs after Rudolf treated his court as an embarrassment and dispersed the collection. The castle has hosted nothing as strange or as fertile since.

St Vitus Cathedral was not finished until 1929, for the millennial anniversary of the death of Saint Wenceslaus. Six centuries from foundation to consecration. The castle visible above the river is therefore a layered structure: medieval Bohemian foundation, Gothic and Renaissance superstructure, eighteenth-century Habsburg facade work, twentieth-century cathedral completion, twenty-first-century working presidency. It is a working monument, not a museum monument, and that is the rarer thing.