Heimburg Castle, Medieval castle ruin in Blankenburg, Germany.

Heimburg Castle, also called Altenburg or Alteburg, is a ruin on the oval Altenburg hill above Heimburg in Blankenburg, Saxony-Anhalt. Its inner ward measured about 35 by 60 metres. Remnants include a curtain wall up to 10 metres high, other wall sections, ditches, ramparts and the round bergfried.

The structure was first mentioned in 1073 under Emperor Henry IV and experienced destruction and rebuilding in its early years. It represents an important phase in the development of fortifications in this region during the early medieval period.

The ruins display medieval defensive elements that speak to the construction methods of the 11th century. Visitors can directly observe and understand how the fortress was once organized and protected.

The ruin lies on a hiking path and can be explored on foot, with good views across the surrounding landscape. Visitors should expect uneven ground and open structures, so caution is needed when entering.

The site served as checkpoint 84 in the Harzer Wandernadel hiking network and acted as an important landmark for walkers exploring the region. This made it a familiar stop for people seeking to discover the mountain landscape and its routes.