Ruine Engelburg, Medieval castle ruins in Mühlhausen, Stuttgart, Germany.

Engelburg is a medieval castle ruin located on a hilltop in Stuttgart's Mühlhausen district. The remaining stone walls show the solid construction methods used to create a defensive stronghold.

The site is probably where the Biberburg of the Alemannic duke Gotfrid stood, documented around 700; in 708 Gotfrid granted the settlement of Biberburg to the church of St. Gallus. A new castle was built here between 1260 and 1280, likely by the lords of Blankenstein. It was destroyed by the Esslinger during the Reichskrieg of 1311–1312 but rebuilt, survived the Thirty Years' War, was still intact in the 1680s and decayed in the early 18th century.

The name may come from Engelbold of the Kaltental family, who introduced the Reformation in Mühlhausen around 1567; the castle was earlier called Biberburg, after a settlement documented in 708. The small castle covered only 480 m², with walls 1.8–2.2 m thick.

The site sits on a hilltop that is reachable on foot with a moderate walk from nearby parking. Wear sturdy shoes as the path crosses grass and uneven ground.

The castle's small size is explained by its type: a Kastellburg, a compact fortification with walls 1.8 to 2.2 m thick enclosing only 480 m². Its moat, the Halsgraben, was filled in during the 1950s, so the ditch that once cut the spur off from the ridge is no longer visible.