Built at the beginning of the 2nd century AD, the Selley Theatre could accommodate around 12,000 spectators, making it one of the largest buildings of its kind. It was modified in the Gallo-Roman age. Both the orchestra and the cavea (tiered seating) are more than semicircular, the rather modestly sized stage building was built onto the exterior of the linear façade wall, and the actual stage consists of a simple wooden platform which reaches into theorchestra.
In the latter part of the 3rd century i twas finally turned into a fortified retreat enclosed by a defensive ditch and was used as such until the mid-4th century.
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.