Karlštejn is perhaps the most remarkable Czech castle, and is one of the symbols of the Czech Kingdom. Charles IV, Czech king and Roman emperor, founded the castle in 1348 on three levels. The lowest floor entailed secular residential rooms occupied by the emperor, his wife and his company. On the second floor, the Church of Our Lady was built, which was connected to the private chapel of Charles IV by a narrow corridor. The chapel was devoted to St. Kateřina, his patroness. The second floor symbolized the place of catharsis. The highest place was the prismatic tower with the biggest sacral space of the castle – the Chapel of the Holy Rood, symbolizing Heavenly Jerusalem. The castle was intended to have been a place of relaxation for its founder, but during the more than seven years of its construction, the idea of its function gradually changed. Finally the crown jewels and relics of the crucifixion were housed here. In the period of Charles´ reign, the castle was predominately a representative seat. From the castle it took one day by horse to reach Prague, where the European political elite would often meet.
In the course of the Hussite wars, Karlštejn was several times the target of attacking Hussites. In 1422, its fortifications proved their quality during an unsuccessful siege by Praguers. In the late 15th century, some Late Gothic adjustments were made; the rebuilding of the Great Tower changed the silhouette of the castle. In the Rudolphinian period of 1578–97, extensive reconstruction in the Renaissance style took place. In 1625, Emperor Ferdinand II abolished the status of Karlštejn and changed the castle and its dominion into the dowry of Czech queens. Maria Theresa donated the dominion to the Institute of Noble Women. The historic meaning of Karlštejn was appreciated duly only in the late 18th century. The purist "regothization" of the castle was decided on and initiated in 1887 under the guidance of architect Josef Mocker. Some of the castle buildings were taken down; some newly built. In 1910, the castle obtained its present-day appearance and was opened to the public.
The prismatic Great Tower with the Chapel of the Holy Rood, consecrated in 1357 and the most expensive part of the complex, dominates the castle. The vaults are decorated with semi-precious stones, set in the shape of crosses, and the unique collection of ideal portraits of saints, popes and bishops, rulers and spiritual teachers. Above the altar there is a niche closed by golden bars where the crown jewels and relics were placed. The vault also shows the sky with the stars, the moon and the sun and the five planets that were known at that time.
The Chapel of the Holy Rood is open to the public, but it is necessary to reserve a tour ahead of time.