Leopold III Friedrich Franz ruled a small, poor state on the middle Elbe and had ideas about what a ruler was for. He travelled with his architect and friend Friedrich Wilhelm von Erdmannsdorff to England and Italy, and the two came back convinced that the Baroque garden - clipped, symmetrical, an argument for absolute order - was finished. They had seen Stourhead in Wiltshire and Ermenonville in France, where the landscape was allowed to look like landscape and the visitor walked through a sequence of moods rather than down an axis. From 1765 Leopold began reshaping his country accordingly. Schloss Wörlitz, built between 1769 and 1773, is generally reckoned the first neoclassical building in Germany. The park around it grew to 112 hectares. The Garden Realm as a whole - parks, palaces, villages, dykes, farmland and river meadow - covers 142 square kilometres, and the point was never the garden alone. The point was that a whole territory could be designed and improved together.

Walk the paths and you walk through a catalogue of the eighteenth century's enthusiasms, each one built properly rather than as scenery. The Gothic House, begun by Erdmannsdorff in 1773 and worked on until 1813, took its model from Horace Walpole's villa at Strawberry Hill and was among the first Neo-Gothic buildings on the continent; one facade is Venetian, the other Tudor. There is a Pantheon of 1795. In 1791 an iron bridge went up here - cast iron, the first on the European mainland, an industrial technology imported as a garden ornament barely a decade after the Iron Bridge opened at Coalbrookdale. Later the taste spread into the villages: Neo-Gothic churches at Riesigk in 1800 and Vockerode in 1811. At Oranienbaum, a Dutch Baroque palace of 1683, Leopold rebuilt the park from 1780 in the Chinese manner set out by Sir William Chambers, with arched bridges, a tea house and a pagoda, and added an orangery in 1811 that runs 175 metres and still shelters citrus trees.

In 1789-90 Leopold built a synagogue in Wörlitz, modelled on the Temple of Hercules Victor in Rome, and placed it where it could be seen across the water from the church. That siting was the argument: the Jewish community of a small German town given a building of the same architectural dignity as everyone else's, in the same view, paid for by the sovereign. It was one of the clearest statements of Enlightenment tolerance any German ruler made in stone. In November 1938 the interior was destroyed in the pogrom. The building itself survived only because the director of the Wörlitz gardens, Hans Hallervorden, intervened to stop it being burned - an argument, presumably, about heritage rather than about the people the building belonged to, and it saved the walls if nothing else. It stands today, restored, still visible from the church, still making Leopold's point.

Leopold's other radicalism was the simplest. In his own lifetime the park was open to visitors - all the buildings, the palace included - at a time when a prince's garden was ordinarily a private stage for a private court. Anyone could come and see how the ideas worked. His brothers and family built outward from the same principle: the Georgium north of Dessau from 1780, whose Anhalt collection holds Dürer's Melencolia I and work by Cranach the Elder; the Luisium of 1774-78, a plain neoclassical country house he gave his wife Louise, with a Neo-Gothic Snake House and an orangery borrowing from Kew; Großkühnau of 1780 at the western end, now the seat of the trust that runs the whole realm. Leopold died at the Luisium on 9 August 1817, of the consequences of a riding accident.

A designed landscape twenty-five kilometres long is vulnerable in ways a building is not. In the 1930s a railway line and the Bundesautobahn 9 were driven through, cutting the realm into four pieces, and the Elbe still floods the meadows the design was built to accommodate. UNESCO inscribed the Dessau-Wörlitz Garden Realm as a World Heritage Site in November 2000, and ICOMOS noted at the time, drily and accurately, that "the overall structure of the landscape has undergone a good deal of deterioration." What survives is still coherent enough to walk. It sits inside the Middle Elbe Biosphere Reserve, so the river meadows and old oak stands that Leopold treated as part of the composition are now protected as habitat, which is roughly what he thought they were.