San Nicandro castle as we see it today dates from the 15th century although it was built on the site of a much more older fortress dating from the Norman and Swabian periods.
The castle is trapeze shaped with four corner towers, and was renovated during the 17th century. On the north side there are two square towers, one of which is one of the original towers, while on the south side there are two round towers built by the Aragonese which are behind the church.
The castle, which was actually built as a mansion, was the usual residence of the Della Marra and all the vassals of San Nicandro Garganico until the time of the Cattaneo Princes. In the 16th century the castle was integrated with the gateway to the east and the lodge was built that connected the castle with a large palace built between the door and the walls of the castle.
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.