The Church of the Holy Trinity, since February 2008 Holy Trinity Cathedral, was built around 1400. The Žilina castle is assumed to be already there as early as the 13th century, of which there are documents from 1318 to 1454. It was originally consecrated to Mary, but in the 16th century it was reconsecrated as the Church of the Holy Trinity. The chapel of John of Nepomuk was added in 1762. The church burned down three times, in 1678, 1848 and partly in 1886. The three naves of the church were originally in Gothic style, but after a reconstruction it was styled to Renaissance style. The last major reconstruction of the church was done in 1942.
The main altarpiece in the main altar depicts the Holy Trinity, the side altars depict Immaculate Conception and the Crucifix, and near the entrance there is a picture of Saint Anne. In the compounds of the church stands separate Burian's Tower, built in the first half of the 16th century. The tower offers a good view of the preserved medieval part of the town.
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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.