The origin of the castle dates back to the 14th century, when Beneš of Vartemberk had a small wooden fortress with a defensive stone tower built on a rocky promontory in the middle of three valleys. A popular legend says that the castle was named Kost during the time when it was besieged by Jan Žižka and, after the unsuccessful conquest of the castle, he declared that it was solid as a bone. However, the truth remains that the founder of the castle, Count Beneš, signed the founding charter with the Latin nickname Benesius de Costi, which the Czechs distorted during the reading and renamed it Beneš of Kost (bone).
After Beneš’s death, his son Petr, who enjoied a high position at the court of Charles IV, became the owner of Kost. Thanks to his position, he was in a very good financial situation and began the overall reconstruction of the rather small fortress into a larger stone castle. The original castle´s core has been preserved to this day.
In 1414, the granddaughter of Peter of Vartemberk was married to the rich Catholic Mikuláš Zajíc of Házmburk. Their descendants remained here practically throughout the entire 15th century. The last owner from Zajíc family was Mikuláš’s grandson, who had appointed guardian Zdeněk Lev from Rožmitál due to his minor age. He sold castle Kost and the estate to Jan of Šelmberk to pay off the debts of the Zajíc family.