Ruine Frankenberg, Medieval fortification ruins in Schmalkalden, Germany.

Ruine Frankenberg is a spur castle on the Schlossberg at 350 m above sea level, 500 m north of Helmers in the lower Rosagrund. Its core is protected by a deep neck ditch and a triple ring wall; the square keep, 7.35 m per side, is built of large red sandstone blocks with putlog holes and has its original high entrance on the southwest side at a height of 11 m. The palas, about 11 m long, adjoins the northeast side of the tower.

Built around the end of the 11th to the beginning of the 12th century, the castle was first documented in 1137, when it belonged to the Lords of Frankenstein. It protected the trade road from Frankfurt via Dermbach and Schmalkalden to Erfurt. In 1293 the male line died out, and the Counts of Henneberg-Schleusingen took over the castle with the consent of the Hersfeld abbots. In 1525 farmers set it on fire.

The castle's lords took their name from it: Otto, documented in 1180, called himself Otto von Frankenberg, and the family held estates at Altenbreitungen, Breitenbach and Fischbach. The line died out in the male branch by 1293, after which the counts of Henneberg-Schleusingen took over the castle. Today a local heritage group, founded in 1983, maintains the ruin.

The ruins are freely accessible and can be reached from the Werra-Burgen-Steig hiking trail, which connects multiple castles in the area. The walk up is moderate with regular rest points and viewpoints along the way.

The keep's lower section is built of large reddish sandstone blocks with Lewis holes, and its original high entrance lies on the southwest side at a height of 11 metres. The tower has a square ground plan with 7.35-metre sides. A staircase was installed inside the tower after 1988, when a test excavation probed the castle courtyard.