Alte Niedeck, Hill castle ruins in Gross Lengden, Germany

Alte Niedeck is a high medieval hill castle ruin on the steep southern summit of the Kronenberg. Its plateau measures about 45 by 30 metres, reached by a path climbing the east slope and turning through the former gate at the southeast corner. A rampart runs around the summit except in the east, surviving only as a terrace in the west and south, with an inner ditch on the gentler northwest and north slopes. An elongated terrace to the east held the outer bailey, whose rampart and ditch extend about 100 metres south.

Built around 1220, the castle served as a residence for the Lords of Niedeck until 1318, when conflict damaged the structure. It formed part of a broader defense network for the region.

The name Niedeck passed to the lords who held the castle, and the site later gave its name to two successive administrative seats: the abandoned hilltop became the "Alte Niedeck", while the manor built below it in the deserted village of Rode was developed from 1616 as the "Neue Niedeck" and new administrative centre. The castle was also the seat of a Vogt and of a court.

The approach path climbs the eastern slope from the north and turns through the former gate at the southeast corner onto the plateau. The rampart on the summit is clearest in the east; in the west and south it survives only as a terrace edge. The forecourt's rampart and ditch run about 100 m south from the east slope of the main castle.

The castle was also the seat of a Vogt and a court, and shares of it were frequently pawned to other owners alongside the main pledge-holders. The eastern terrace, about 100 m long, is where the outer bailey buildings are thought to have stood.