The terms were generous and entirely self-interested. Settlers received freedom of religion, extensive privileges, and a civic constitution modelled on Amsterdam and Leiden; Dutch became the official language of the town. In return the duke expected Dutch money and Dutch expertise - specifically the polder engineering that could turn fen and salt marsh into farmland, and the trading networks that might make his new port matter. It was named, without excessive modesty, after himself. Among the first arrivals was Johannes Narssius. The scheme half-worked and half-collapsed almost at once: by 1630 many of the Remonstrants had already gone home, the political weather in the Netherlands having improved faster than Friedrichstadt's prospects.

What they left behind is the reason people come now. The town was laid out on a checkerboard grid across roughly seventeen hectares of reclaimed ground, split by two canals - the Mittelburggraben and the Fuerstenburggraben - with a market square at the centre. The houses that line them are tall, narrow and gabled, built of Dutch brick shipped in for the purpose in the Backsteinrenaissance manner, their stepped and scrolled gable ends facing the water exactly as they would in Leiden. Trees lean over the canals. A cast-iron water pump from 1879 stands on the market square. Germans call the place the Hollaenderstadt, and visitors reliably reach for a shorter description: little Amsterdam. Friedrichstadt sits on the Eider about twelve kilometres south of Husum, in a landscape so flat that the gables are the tallest thing for miles.

The tolerance clause was not decorative, and word travelled. The Remonstrants built here what is described as the first church anywhere expressly built for their confession. Mennonites came from Eiderstedt and the Netherlands in enough numbers that four separate congregations eventually merged into one in 1708. Catholics were granted toleration in 1624 - remarkable in a Lutheran duchy less than a decade into the Thirty Years' War - and eventually raised St. Knud's in 1854. Quakers put up a meeting house in 1677. The Unitarians had a building of their own on the Mittelburggraben. Ashkenazi Jews arrived from 1675 and grew into the second-largest religious community in the town for around a century. The Lutherans, meanwhile, quietly became the largest congregation of all and built St. Christophorus between 1643 and 1649, where the painter Juergen Ovens, a pupil of Rembrandt and court artist to the Gottorf dukes, is buried.

Friedrich III's larger ambition was to make his town the European terminus of an overland trade route to Asia - silk from Persia arriving without the long haul around Africa. Between 1633 and 1637 he despatched an embassy to Tsar Michael I of Russia and Shah Safi of Persia, led by the jurist Philipp Crusius and the merchant Otto Brueggemann, with the scholar Adam Olearius as secretary. Diplomatically it achieved nothing. Culturally it achieved a great deal: Olearius came home and wrote an account of the journey that long outlived the mission itself. The trade route stayed hypothetical, the town stayed small, and it has stayed small ever since - about 2,500 people today, which is very likely fewer than the duke had in mind. Louis Philippe, the future king of the French, spent a few months here as a tutor while in flight from the Revolution, which tells you what sort of place it had become: quiet, and good at not asking questions.

Twice the arrangement broke. During the First Schleswig War, from 29 September to 4 October 1850, Friedrichstadt was bombarded; 31 residents were killed or wounded and 285 buildings destroyed, among them the town archive and the Remonstrant church, which was rebuilt between 1852 and 1854. The second loss was not architectural. The Jewish community that had lived here since 1675 numbered around 500 people at its height in the mid-nineteenth century; by 1933, after generations of migration to the cities, 32 remained. On the night of 9 to 10 November 1938, SA men destroyed the synagogue, and Jewish men of the town were arrested, three of them sent to Sachsenhausen concentration camp. The community that helped make Friedrichstadt what it was does not exist any more. Two cemeteries remain, and 25 Stolpersteine set into the pavement of a town founded on the promise that faith would not be held against you.