Nostra Segnora de Mesumundu is located in the archaeological complex with the same name. It was built in the 6th century, during the Byzantine domination of the island, over a pre-existing Roman structure (2nd century AD). The Byzantines re-used part of the walls of the Roman building, as well as the aqueduct. The edifice could have been used as a baptistery. However, it is also possible that it was used for the purification of ill people through an immersion rite.
In 1063, the structure was donated by the Giudice (duke) Barisone I of Torres to the Abbey of Montecassino. When monks came from the Italian religious community, they adapted the building for Roman Catholic use, adding an apse and a new entrance (demolished in 1934). For the work, they used materials from the nearby Roman ruins and the nuraghe Culzu.
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.