Burg Bosselstein, Castle ruin in Idar-Oberstein, Germany

Burg Bosselstein, also called Burg Stein or Altes Schloss, is a ruined hilltop castle on the Kirchenfelsen, a steep rock outcrop above the Felsenkirche and the Nahe valley in Idar-Oberstein. Surviving elements are the stump of the round bergfried, remains of the palas and sections of the ring wall. The site lies next to Schloss Oberstein.

Built in the 12th century by the Lords of Stein, the castle was first mentioned in a document in 1197. It was abandoned around 1600 and fell into ruin. In 1945 the ruin passed to the state of Rhineland-Palatinate and is now managed by the Generaldirektion Kulturelles Erbe.

The castle's name comes from the Lords of Stein, who built it in the 12th century and gave their name to Oberstein; "Bosselstein" is the later form used for the rock spur. The ruin is now a protected cultural heritage monument cared for by the Generaldirektion Kulturelles Erbe of Rhineland-Palatinate. Visitors reach it on foot above the Felsenkirche, and the surviving ring wall and tower stump show how the summit was enclosed.

The ruin stands on the Kirchenfelsen, a steep rock outcrop directly above the Felsenkirche and the Nahe valley, beside Schloss Oberstein. The site is not wheelchair accessible; the approach is on foot and involves climbing. The surviving round bergfried stump, palas remains and ring wall can be reached on foot.

The fortress sits directly above the Felsenkirche, a chapel carved into the living rock, creating an unusual vertical pairing of two structures. This arrangement shows how religious and secular authority occupied the same physical space in medieval times.