The ruins of the Grottenburg (a castle built into a naturally occurring cave) Marmels are located on two rocky points high above the embankment dam that formed the Lai da Marmorera near the municipality of Marmorera.

On the lowest level, there are portions of two sections of the curtain wall which follow the edge of the cliff.

The three-story, square keep is located on the southern terrace and rested against the surrounding rock, as can be seen in the sketch from 1893.

A steep path along the rear wall of the grotto leads to the upper level of the castle site.

In addition to their castle at Marmels, they held Burg Spliatsch as a Vögte (or reeve) and the village of Riom.

The most important member of the line was Conradin von Marmels († 1517/18) who held titles to Haldenstein and Rhäzüns.

His younger son, Rudolf, received Burg Haldenstein, was mayor of Chur and later the primary Landeshauptmann in Veltlin.

The castle remained in the possession of the family von Marmels throughout its history, which is quite unusual.

Conradin's son Rudolf sold the castle, along with the tower of Tinizong and the Burg Spliatsch to his nephew Hans.