Büraburg

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. 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.

Source: Wikipedia