The central tower and oldest part of the Rietberg Castle was built in the 13th century, though it may have been built around an earlier 12th-century tower. It was built for the Lords of Rietberg, who in 1286 were vassals of the Freiherr von Sax-Misox. In the 14th century they became vassals of the Bishop of Chur. At that time they held the castle and estates in Schams, Chur and Oberhalbstein and were the bishop's representative in Oberhalbstein and Oberengadin. The last male member of the Rietberg line, Johann von Reitberg, attempted to guarantee that the castle and his estates would pass to his wife, Berta von Rhäzüns, and to his relative Hermann von Landenberg. However, after his death in 1349 the Bishop of Chur claimed the castle. In 1353 he succeed in forcing the Landenbergs to sign away their claim to the castle and estates. Other claims to the castle continued to simmer until 1388 then the Bishop paid off the Lords of Rhäzüns and Lumerins to settle their claims.
The Bishop used the castle, its estates and the office of vogt as collateral for loans from a number of wealthy families or as a reward for gifts from those same families. In 1384 Eglolf von Juvalt lived in the castle. In 1398 the bishop mortgaged the castle to Jakob von Castelmur for 500 silver marks. It was mortgaged again in 1409 to Conradin Rambach and the cathedral chapter, again in 1426 to Bartolomäus Planta and in 1447 to Hans Wellenberg. In 1450 it was mortgaged to Hans Ringg, who passed it to his son Wilhelm in 1483. It continued to used as collateral into the 16th and 17th centuries. Around 1530 it went to Anton von Travers, followed by Hercules von Salis in 1554. In 1617 it came to the Planta family.
It was the site of the murder of Pompeius Planta in 1621 by Jörg Jenatsch during the conflicts between Catholics and Protestants known as the Bündner Wirren.
References:Celje Castle was once the largest fortification on Slovenian territory. The first fortified building on the site (a Romanesque palace) was built in the first half of the 13th century by the Counts of Heunburg from Carinthia on the stony outcrop on the western side of the ridge where the castle stands. It had five sides, or four plus the southern side, which was a natural defence. The first written records of the castle date back to between 1125 and 1137; it was probably built by Count Gunter. In the western section of the castle, there was a building with several floors. Remains of the walls of this palatium have survived. In the eastern section, there was an enclosed courtyard with large water reservoirs. The eastern wall, which protects the castle from its most exposed side, was around three metres thicker than the rest of the curtain wall. The wall was topped with a parapet and protected walkway.