What went inside was encyclopedic in the literal sense: an attempt to hold the whole world in a set of rooms. The state library of Hesse. The landgraves' art collections, including a Modern Statue Gallery. A room for medals, machines and clocks. A print room, a manuscripts room, a map gallery, scientific instruments. Cork models of Roman architecture, painstakingly built for people who would never see Rome. Wax figures of the historic landgraves themselves, watching visitors inspect their possessions. The library was designed to hold a hundred thousand volumes. By the end of the nineteenth century the clock and watch collection was among the largest in the world - a fitting speciality for a town whose name became bound up with licensed clockmaking. Attached to the museum stood the medieval Zwehrenturm, a surviving tower of the old city wall, converted into an observatory. The building looked outward as well as in.

In 1807 Napoleon carved a Kingdom of Westphalia out of conquered German territory and installed his youngest brother Jerome as its king, with Kassel as the capital. The Fridericianum stopped being a museum and became a parliament building - the first in Germany - which is a strange fate for a collection of clocks and cork models, and a stranger one for a building financed by a prince Napoleon had just deposed. The court needed librarians, and it hired two brothers who had grown up in the region: Jacob and Wilhelm Grimm. When Jerome was driven out in 1813 the building reverted to its original purpose, its brief parliamentary career over almost before it began.

Prussia annexed Hesse in 1866, and what followed was quieter than a fire and just as thorough. The collections were gradually transferred to Berlin, where the power now sat, and the building was hollowed out object by object. By 1913 it had ceased to function as a museum at all, retaining only its status as a state library. Then came the bombing. Kassel was a centre of tank and aircraft production, and Allied raids in 1941 and 1943 gutted the Fridericianum; when it was over, the enclosing walls and the Zwehrenturm were essentially all that was left. A building conceived to show a prince's version of the world had been stripped of its contents by one government and burnt out under another, within eighty years.

The ruin became the opportunity. In 1955 the Kassel artist and curator Arnold Bode filled the patched-up interior with the modern art the Nazi state had condemned and removed from German museums, and called the exhibition documenta. It has returned every five years since, and the Fridericianum has been its centre ever since - a building whose lack of fixed collections, an accident of Prussian removals and RAF incendiaries, turned out to be exactly what a recurring contemporary exhibition needs. Between documentas it runs as an art gallery in its own right, with more than two thousand square metres of exhibition space. Susanne Pfeffer directed it from 2013, mounting the trilogy Speculations on Anonymous Materials, nature after nature and Inhuman, a retrospective for the American experimental filmmaker Paul Sharits, and, for documenta's sixtieth anniversary, a full survey of the Belgian artist Marcel Broodthaers. Moritz Wesseler has led the institution since November 2018, with a programme built around artists who have had no institutional platform in Germany. Two and a half centuries on, the building is still doing what Frederick built it for: putting things in front of people who were not invited.