New Slains Castle, to distinguish it from nearby Old Slains Castle, was originally a 16th-century tower house, built by the 9th Earl of Erroll. The wings around the courtyard were extended in 1664 by addition of a gallery or corridor, and in 1707 the entrance front was renewed.
In the 1830s the 18th Earl commissioned the Aberdeen architect John Smith to remodel the castle. This resulted in a virtual rebuilding of Slains in a Scots Baronial style, including granite facings, in 1836–1837. Gardens were laid out in the late 1890s.
At first inspection the ruin appears to be a blend of several different architectural styles and periods, due to diverse masonry including older mortared granite, mortared medieval red brick, mortared sandstone and newer well faced granite. In fact most of the architecture seems to derive from a rather cohesive interval 1597 to 1664, which construction is the most expansive and includes the mortared rough granite and medieval brick. The 1836 work adds smoother granite facing that contrasts with the older construction style.
The defensive works of the castle include use of the North Sea cliffs; an abyss to the west that functions as a deep impassable moat; and a ruined rampart that would have been the main entrance on the south. The ruins include reasonably well preserved elements of three- and four-storey structural elements and a basement course over some of the range, especially at the eastern side. There are well-preserved basement kitchen works with numerous firepits and masonry indented storage spaces. The internal doorways are primarily of well-preserved wooden lintel construction, with numerous examples of mortared sandstone and medieval brickwork archways. The interior of the ground level is a maze of passageways and smaller rooms, reflecting a high state of occupancy in 17th-century times.
References: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.