Established as a small outpost by the Romans, Nijmegen was defined by the Valkhof, a large stronghold that Charlemagne built in the 8th century on a bluff overlooking the river Waal. The citadel was rebuilt by Emperor Frederick Barbarossa in the 12th century after a fire, but following a period of neglect most of the fortress was dismantled in 1796; only a chapel from the 11th century and ruins of a second chapel remain.