After Charlemagne had removed Saxons from some of their lands and given it to the Obotrites (who were allies of Charlemagne), he finally managed to conquer the Saxons in the Saxon Wars. In 811 he signed the Treaty of Heiligen with the neighbouring Danes and may at the same time have reached a border agreement with the Polabian Slavs in the east. This border should not be thought of as a fortified line, however, but rather a defined line running through the middle of the border zone, an area of bog and thick forest that was difficult to pass through. According to Adam of Bremen's description in the "Gesta Hammaburgensis ecclesiae pontificum" about 1075, it ran from the Elbe river near Boizenburg northwards along the Bille river to the mouth of the Schwentine at the Kiel Fjord and the Baltic Sea.