Old Castle, Cultural heritage monument and lowland castle in Koblenz, Germany.

The Old Castle (Alte Burg) is a former electoral lowland castle on the right bank of the Moselle in Koblenz's old town, beside the Balduin Bridge. Only the Burghaus survives: a three-storey building with a steep slate hipped roof and two tall towers on the river side, topped by Welsh cupolas. Its city side has a hexagonal stair turret and paired rectangular windows, while the lower Moselle front and east tower rest partly on Roman foundations.

The fortress was built around 1185 as a seat of power and incorporated remains of a Roman tower from an earlier time. Centuries later, it was converted into a factory before eventually returning to its role as a historical landmark.

The castle's name, Alte Burg, is still used locally, and the building now serves as the Koblenz city archive, open to researchers. Its west tower once stood free beside the bridge gate of the Balduinbrücke, and the paired rectangular windows and shell-topped gables on the town side are Renaissance details a visitor can pick out.

The surviving Burghaus stands on the right bank of the Moselle, just upstream of the Balduinbrücke in Koblenz's old town. It is a city archive, so the interior is not open to tourists; the building is seen from the Moselle towpath and the bridge approach. The Moselle front rests partly on Roman foundations.

Between 1806 and 1897, the fortress was not a seat of power but a sheet metal factory, an unusual industrial period in its history. This conversion shows how old structures were sometimes repurposed for entirely different uses.