Burg Arnsburg, Medieval castle ruins in Lich, Germany.

Burg Arnsburg is a ruined spur castle on a ridge 800 metres southwest of Kloster Arnsburg, 17 metres above the river Wetter. Its remains include a square tower surrounded by a narrow courtyard wall and a 17-metre-deep dry-stone well reaching the groundwater. The foundations were excavated in 1984–1985 and rebuilt up to above ground level.

A first castle stood near the abandoned Roman fort around 800, and a second was built around 1000 on a spur above the Wetter. It was the ancestral seat of the von Hagen und Arnsburg family, who moved to Münzenberg after 1156. Monks then reused its stone for the nearby monastery, and the ruins were excavated and consolidated in 1984–1985.

The name Arnsburg has been read as "castle of Arn" or "castle of Arnold", though these names do not occur among the later lords; a link to the Roman legionary eagle is also proposed, since Kuno von Arnsburg kept the eagle as his heraldic animal. The site is now a field of excavated and partly rebuilt foundations on the Hainfeld above the Wetter.

The ruins are accessible via marked hiking trails with good viewing points across the surrounding area. Wear sturdy footwear as the ground is uneven in places and some paths slope steeply.

Beneath the castle walls lie traces of earlier settlements showing human occupation long before the medieval period. Archaeologists continue to uncover artifacts that reveal how this elevated location was used across many centuries.