The Castle of Monopoli, surrounded by the sea and located on a peninsula, was built first as a stronghold and later changed into noble house. It was originally built as part of coastal strongholds by Holy Roman Emperor Frederick II and later fortified by the Angevins. In 1552 it was once again reinforced by the viceroy Don Pedro de Toledo, at the behest of Charles V. Other important changes were made in 1660, by the Duke of Atri Giovanni Geronimo Acquaviva.
The present appearance dates back to 1660, when the huge cylindrical keep was added, from which it is possible to enter. On the upper part, there are large holes for the cannons, a sign of the original function to protect the town, while the right part, towards the town, appears to be more refined after the works of the 17th century, when it started to be used as house.The castle, reshaped several times over the centuries and used as prison during the first years of the 20th century, was restructured around 1976 and nowadays houses the Archaeological Museum 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.