St. Nicholas' Cathedral is a significant, well preserved example of northern German Late Brick Gothic; the church features three aisles, two transepts and a single-aisled ambulatory. Particularly impressive is the church's northern tiered gable. The church features 22 stained glass features considered to be treasures of international significance, a feature of the cathedral elevating the building to a heritage site of Europe-wide importance.
The cathedral was founded in 1188 by Margrave Otto II. The construction of present building was begun in 1423. In World War II the cathedral suffered damages during an air raid on April 8, 1945. The vault of the south transept and parts of the cloister were destroyed. The medieval windows had been removed before and survived.
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.