Grm Castle is located on a hill on the southern outskirts of Novo mesto. It was built in 1586 by Krištof Mordax, a nobleman from an old Carniolan-Carinthian-Styria family, who had previously owned the Graben ob Krki manor house near Novo mesto. The castle originally consisted of a square tower and a ground floor wing. Due to the danger of Turkish attack, the tower also had a defensive and observation function in addition to its residential function. To the right of the tower, the originally ground floor wing expanded. In 1636, after Krištof's death, the castle was taken over by his son Hans Andreas, who renovated the building for the first time and had another floor added to the right wing. After his death, the castle was taken over by his son Wolfgang, who changed the appearance of the castle the most. He had the northern wing to the left of the tower added and a high stone wall on the south side. The spacious courtyard facade was then given wide open corridors with stone columns above the ground-floor arcades, which were later walled in. The castle thus acquired a rectangular shape with a central courtyard in its ground plan, thus taking on the appearance of late Renaissance and Baroque castles.
At the beginning of the 18th century, a castle park was designed, connecting the castle with the Chapel of the Holy Sepulchre, which is also the mausoleum of the Mordax family and is located on a nearby hill. In the 1880s, the castle was taken over by the Land of Carniola. It housed the newly established agricultural school, which was moved to Grm from Slap Castle near Vipavi. When it was founded, the school was called the Wine, Fruit and Agricultural School at Grm near Rudolfov. The school operated in the castle from 1886 to 1990, when it was moved to Bajnof below Trška Gora. The Revoz company then set up its premises in the manor house.
The castle was declared a cultural monument of local importance by the Decree on the Proclamation of Natural Landmarks and Immovable Cultural and Historical Monuments in the Municipality of Novo Mesto, Official Gazette of the Republic of Slovenia No. 38/1992, and has been intensively renovated in recent years. Today, it houses the Historical Archives and the Institute for the Protection of Cultural Heritage of Novo Mesto. Various social and cultural events are also held there in the summer.
The castle was already mentioned by Janez Vajkard Valvasor, who mentioned in his notes that the castle had richly painted rooms. Only the painting in the central hall of the castle tower has been preserved in its entirety to this day. The ceiling of the hall is painted with military scenes, which are framed with richly articulated stucco frames from the 17th century, and paintings with military scenes of battles with the Turks are placed in it. In the middle is a field with the coats of arms of the Mordax and Gallenberg families.
The castle houses the Institute for the Protection of Cultural Heritage and the Novo Mesto unit of the Historical Archives of Ljubljana. It appears that the castle will be renovated.