The Upstalsboom is the only known meeting place where representatives of all Seven Sealands of medieval Frisia actually convened. Other assemblies existed within individual sealands, but the Upstalsboom drew together emissaries from across the Frisian-speaking coast - the lands between the Lauwers River in the west and the Weser estuary in the east, and eventually farther afield as the Central Frisians joined and as Groningen was forced into the league after the Siege of Groningen in 1338. Each sealand sent one redjeven, who served for one year. The decisions reached at the annual meeting had legal force across the participating communities. Comparison has often been made to the Icelandic Thingvellir, the open-air parliament where Iceland's Althing met from the tenth century onward. The Frisian version was smaller and less institutionalized, but it ran on the same principle: a culture without a central authority needed a meeting ground that everyone could agree was neutral.

On 18 September 1323, the Statutes of Upstalsboom were ratified in Central Frisia in an attempt to revive a league whose legal force was slipping as the Count of Holland pressed his claims into Frisian territory. Three versions of the Statutes survive. Two are in Old Frisian, one in Latin. The Latin version, twenty-four paragraphs long, is believed to be the original. One Old Frisian translation also has twenty-four paragraphs; the other has thirty-six. The Statutes are one of only a handful of legal texts that exist in both Old Frisian and Latin, which makes them important to philologists as well as historians. The 1323 punishments were considered extreme even for the period. When the statutes were reaffirmed in 1361, some of them were scaled back - though one new clause added a forty-mark payment to the heirs of a homicide victim by anyone who concealed a murder across sealand boundaries, twice the usual weregild. The text reads like the work of a coalition of independent communities trying to hold lawlessness at bay with whatever leverage they had.

The hill at Upstal was sacred ground long before it became an assembly site. Archaeological evidence shows that members of important Frisian families were buried here as early as the eighth century, hundreds of years before the league is first attested in writing around 1220. By the time the redjevens started meeting here, they were standing on top of bones. That layered significance is part of what made the site usable as a political center: the dead were already there, and the ancestors gave the proceedings a weight no royal charter could provide. The earliest assemblies dealt with payment of fines to local counts. By the thirteenth century the league was caught up in a complicated war between two villages in the Ommelanden, which dragged in Fivelingo, Hunsingo, Groningen, and others. The Upstalsboom League was politically successful and militarily weak. It could not enforce its own agreements when communities defied them - one of the persistent frustrations its statutes try to address.

The Frisian freedom period was marked by extraordinarily successful repulsion of foreign invasion. Other than two notable defeats - in 1270 against the Count of Stotel, and in 1289 against the Count of Holland - the sealands consistently beat back the armies that came to subjugate them. They did this without a king, without a standing army, and without the central tax authority that would have made permanent military forces possible. They did it through what contemporary observers called Frisian solidarity, a willingness to fight together that did not require a sovereign to coordinate it. In 1338 the Upstalsboom League entered a military alliance with France against England and the Count of Gelre. By the late Middle Ages the system was breaking down. The growth of Groningen, internal violence, and the rising power of regional chieftains together undid the equilibrium that the annual meeting had once preserved. The Statutes were the league's high-water mark and also the early sign of its decline.

In 1883, German nationalism in its romantic-historical mode reached the Upstalsboom. A stone pyramid was erected at the site to memorialize the Frisian freedom period - a modest monument by the standards of the era's commemorative architecture, but a deliberate gesture. The pyramid still stands. Tourists who visit sometimes take handfuls of soil home in small bottles, treating the ground as something between a historical landmark and a relic. The hill itself is unimpressive at first glance: a low rise in the flat East Frisian Geest, easy to miss if you don't know to look. What it remembers is harder to miss once you do - a thousand years of a coastal people who, against all the structural odds of medieval Europe, organized themselves around a tree, an annual Tuesday, and the simple shared belief that they were free.