Burgruine, Medieval castle ruins in Zeil am Main, Germany.

Burg Schmachtenberg is a ruined hilltop castle on a spur above Zeil am Main, its core surrounded by a zwinger with four corner towers, of which two round towers and three sides of the ring wall survive. The plateau forms an irregular trapezoid of 56 by 42 metres, cut off on the east by a 15-metre-wide neck ditch. A vaulted cellar and the foundations of the north domestic buildings were excavated after 2008.

The fortress was built around 1430 and suffered damage during peasant uprisings in 1525 and 1554. It was finally abandoned during the Second Margrave War and gradually fell into decline.

The castle was built around 1420/30 to replace the older Zeil castle on the Käpelle, and its construction went hand in hand with the expansion of Zeil's town fortifications, as shown by identical wall-walk designs. The name Schmachtenberg refers to the hilltop site above Zeil am Main.

The ruin stands on a plateau above Zeil am Main, cut off on the east by a neck ditch about 15 metres wide. The surviving north-west tower and the long stretch of curtain wall on the river side are the easiest parts to make out; the south-west corner tower is gone entirely. The ground inside the walls is uneven and partly overgrown, so wear sturdy shoes.

The castle was built around 1420–30, probably to replace the older Zeil castle on the Käpelle, and its embrasures were already designed for firearms. Its construction evidently went hand in hand with the expansion of Zeil's town walls: both defences share identical wall-walk constructions. The spur castle was damaged by rebellious peasants in 1525, repaired, and devastated again in 1554 in the Second Margrave War.