The mighty St. Gallus church from the 17th to 18th century sits enthroned above the city. The church was constructed on a pre-Christian cult site and was redesigned on multiple occasions by the famous baroque master builders of the Bregenz Forest.
The church was built to the site of pre-Romanesque church founded around 450 AD. Irish monks Columbanus and his brother Gallus founded the destroyed church in 610 AD and ordered to rebuild it.
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.