The Büraburg was a prominent hill castle with historic significance, on the Büraberg hill overlooking the Eder river near the town of Fritzlar in northern Hesse (Germany). Only foundation walls remain, and a church dedicated to St. Brigida. A Christian monastery was located on the hill already in the seventh century. It was occupied by the Franks c. 690, who fortified it with a castle and kept a mounted garrison there. The Franks supported the missionary efforts of Saint Boniface, who cut down the Donar Oak in nearby Geismar and established a short-lived bishopric in Büraburg. Background Strategic importance Hessia, from the early seventh century on, served as a buffer between areas dominated by the Saxons (to the north) and the Franks who brought the area to the south under their control in the early sixth century and occupied Thuringia (to the east) in 531. By 650, the Franks were establishing themselves as overlords, which is suggested by archeological evidence of burials, and were building fortifications in various places including Christenberg. By 690 they were taking direct control over Hessia, apparently to counteract expansion by the Saxons, who built fortifications in Gaulskopf and Eresburg across the river Diemel, the northern boundary of Hessia. Büraburg (which already had a Frankish settlement in the sixth century) was one of the places the Franks fortified in order to resist the Saxon pressure, and according to John-Henry Clay the Büraburg was "probably the largest man-made construction seen in Hessia for at least seven hundred years". Walls and trenches totaling one kilometer in length were made, and they enclosed "8 hectares of a spur that offered a commanding view over Fritzlar and the densely populated heart of Hessia". The presence of a mounted garrison is deduced from archeological finds, including spurs, harness fittings, and fragments of weapons. During the Saxon wars of Charlemagne, it was an important fortified base of the Franks, but with the final submission of the Saxons in 804 it lost its military significance. Religion The area around Fritzlar shows evidence of significant pagan belief from the first century on. Geismar was a particular focus of such activity; it was continuously occupied from the Roman period on, with a settlement from the Roman period, which itself had a predecessor from the fifth century BC. Excavations have produced a horse burial and bronze artifacts. A possible religious cult may have centered on a