Otto II ordered the two sides to cease fighting, travelled to Groningen and arranged a truce. When the truce was violated, Otto repeated his demands for peace and returned to Utrecht to raise an army. In his absence, Egbert built a line of defences at Glimmen, southeast of Groningen. Rudolph considered it a provocation, but Egbert defended his right to build whatever fortifications he wished within his own jurisdiction. The Drenthers nonetheless attacked Glimmen, razed the defences and took many prisoners, forcing Egbert to retreat into Frisia. There he recruited Frisian allies and marched on Groningen, which was occupied by Rudolph's forces. After serious fighting, Egbert retook the city and forced the occupiers to retreat to Drenthe.