The castle complex on the Willibaldsberg was begun in 1355 and extended in the second half of the 16th century under Martin von Schaumberg. It was transformed into an impressive residence by Elias Holl during the reign of Prince-Bishop Johann Conrad von Gemmingen (1595-1612) – at this stage of the building's history the towers were crowned by onion domes.
Gemmingen also laid out the renowned botanical garden Hortus Eystettensis. Based on the copperplate engravings illustrating the plant collection which were created in 1613 by Basilius Besler, the Bastion Garden opened in 1998 reproduces the plant world of the original botanical garden.
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.