Altes Schloss, Medieval castle ruins in Kleinwallstadt, Germany.

Altes Schloss Kleinwallstadt is a medieval castle complex whose stone walls rise in places to heights of about 10 meters (33 feet) and clearly show the outline of the former structure. The ruin sits in a forested area and consists of several foundation sections that indicate the castle's original size and layout.

The castle was built shortly before the mid-13th century, first documented in 1230 as Burg Waleberg after its acquisition by Kurmainz; a Konrad von Waleberg named himself after it from 1225. Excavations show it was destroyed deliberately in the second third of the 13th century, before completion, by fire and by undermining its walls, during conflicts between Mainz and the lords of Rieneck. By 1594 it was already shown as an overgrown ruin.

The ruins reveal traces of a well-organized medieval fortification that visitors can still observe in the remaining stone foundations. Walking through the site offers insight into how the structure once functioned as a defensive complex.

The ruin is located in the forest and accessible by marked hiking trails. Wear sturdy footwear and expect uneven ground when exploring the site, as the terrain around the ruins can be rough.

The castle was never finished: excavations in 2006–2010 showed its half-timbered interior was deliberately set on fire and the walls undermined in the second third of the 13th century. The site was long misidentified as Burg Waldenberg, which actually stood on the Klosterberg near Hösbach-Rottenberg.