The Round Tower is 86 feet high, with a 50-foot circumference at the base and a 16-foot diameter. It is capped by a hexagonal spire added in the 14th century. The narrow single doorway is raised several feet above ground level in the manner typical of Irish round towers, designed so that the ladder could be pulled up after the occupants. What makes Brechin's tower exceptional is the carving. The arch above the door bears a well-preserved Crucifixion scene. The splayed sides have relief sculptures of two ecclesiastics: one holds a crosier, the bishop's staff, while the other carries a distinctive Tau-shaped staff. The doorway is enriched with two carved bands of pellets. The masonry is superior to all but a very few of the Irish examples. Someone, around the year 1000, brought a piece of Ireland to Brechin and built it to last.

Inside the cathedral are two further survivals of that early medieval moment. The Brechin Hogback is an 11th-century carved gravestone of a particular Viking-period type - a long curved stone meant to mimic the roof of a longhouse - that combines Celtic and Scandinavian decorative motifs. It is the most complex known piece of sculpture in the Ringerike style anywhere in Scotland. The Ringerike style takes its name from a region of Norway and reflects the cultural mixing of late Viking-age Britain. Beside it stands St Mary's Stone, an inscribed cross-slab whose central Virgin and Child are framed by a circular border that echoes the carved decoration on the Round Tower itself. Both stones are rare and important. Almost nothing else like them survives in Scotland.

The cathedral itself dates from the 13th century, built in the Pointed Gothic style. In 1806 well-meaning Victorian restorers got at it with results so poor that another restoration in 1902 had to undo most of their work. What survives from the original is the western gable with its flamboyant window, the Gothic doorway and massive square tower, parts of the much-shortened choir, and the nave pillars and clerestory. The modern stained glass in the chancel is regarded as among the finest in Scotland. After the Scottish Reformation the cathedral lost its Catholic function and became a Church of Scotland parish church - which it remained for over four hundred years.

In February 2020, the Presbytery of Angus voted for a dissolution motion. Ownership of Brechin Cathedral passed to the General Trustees of the Church of Scotland, who proposed to close and sell the building. The committee planning the 800th anniversary celebrations on the 7th of June 2020 had to abandon them when Covid restrictions arrived. The cathedral closed its doors for the final time as a sanctified church on the 28th of November 2021, after roughly eight centuries of worship. Almost immediately a new committee of Trustees was established, led by Caroline Carnegie, Duchess of Fife, to take over care of the building and try to keep it as a focal point for the town. The Round Tower still stands. The hogback and St Mary's Stone are still inside. What happens next is being written, slowly, by the people of Brechin.