Alte Burg, Castle ruin in Burgsinn, Germany.

Alte Burg, also called Altes Schloss, is the ruin of a water castle just northeast of Burgsinn's centre. A broad moat fed by an underground spring surrounds a trapezoidal plan whose corners are marked by towers; the ring wall is of red sandstone bossed ashlars, other walls of rubble stone, and the southern residential building carries timber framing on its upper storey. The square five-storey bergfried rises 22 metres, with sides of 8.50 metres and a high entrance 8.50 metres up.

The site was held by the Barons of Thüngen; in 1001 Hildolf von Tüngen exchanged the village of Sinna and its appurtenances with the Bishopric of Würzburg. In 1303 Gozo, Aplo and Hermannus de Sinna resided at the castle, which became a fief of the Counts of Rieneck in 1317. In 1337 the Bishop of Würzburg granted it as a castle fief to Dietz von Thüngen, who built the present castle essentially between 1339 and 1342 and sold it with all possessions to his son Wilhelm von Thüngen in 1405.

The name Alte Burg (Old Castle) distinguishes the ruin from Burgsinn's free-hold Schloss, which passed to the Andreas line of the Thüngen family and is now unoccupied, its owner Gottfried von Thüngen living in France. The moat is fed by an underground spring, and the southeast tower of 1543 is attributed to Martin I. von Thüngen, who built much on the old castle.

The moat around the castle is fed by an underground spring, so the ground near the water can be soft. Access to the interior is from the west side over a bridge; the site lies just northeast of Burgsinn's centre, about 200 metres west of the Sinn. The five-storey bergfried is 22 metres high, and its high entrance sits 8.50 metres above the courtyard.

The five-storey bergfried is square, not round: 22 metres high, 8.50 metres on each side, built of bossed sandstone blocks in the 12th century, with its high entrance set 8.50 metres above the courtyard. The surrounding moat is fed by an underground spring.