Carlow Castle has a large rectangular three-story limestone keep with circular towers at each angle. The castle dates from the early 13th century. It was built by William Marshal the elder in the time period between 1207 and 1213 which he spent in Ireland. The castle in Carlow was the very first of its kind in Ireland.
The castle was handed over to the crown in 1306, granted in 1312 to Thomas Plantagenet, confiscated by the crown in 1537 as the landlords were absent, bought by the Earl of Thomond in 1616, changed hands multiple times until it was taken by Oliver Cromwell in 1650 but was later returned to the Earl of Thomond. In 1814 the castle was widely destroyed in an attempt to create more space for the conversion into a lunatic asylum with the help of explosives. Just the outer face of the west wall and the two neighbouring towers could be preserved.
The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.