Martinsburg, Medieval château in Lahnstein, Germany

Martinsburg is a medieval castle in Lahnstein, Rhineland-Palatinate, standing near the point where the Lahn river meets the Rhine. The structure consists of a rectangular tower house with two corner towers and today serves as a mix of private apartments, offices, and a museum.

The castle was built in 1298 as a customs post for the Archbishops of Mainz to control river traffic on the Rhine. The tall keep was added around 1400, significantly expanding the structure.

The castle's name recalls Saint Martin, though the sources do not explain the change from its original name, Burg Lahnstein. Since 1803, when it ceased to belong to Kurmainz, it has passed into private ownership; today it holds apartments, offices, a carnival museum and a minting house. Visitors see a private property, not a public museum.

Martinsburg is a private property with apartments and offices, so the interior is not open to visitors and no guided tours are offered. The building can be viewed from the outside, from the Rhine bank and from the town side, where the moat, the gate with its machicolation and the 28-metre keep are visible.

The keep was built around 1400 with an integrated spiral staircase and toilet facilities inside the walls, which was very rare at the time. This shows that the people who designed it were already thinking about everyday comfort, not just defence.