A round tower is the earliest structure surviving from Brechin cathedral. It was originally detached, but was integrated into the church in 1807 when a doorway on the N side of it was inserted. This doorway was subsequently blocked about 40 years later. Nothing of the 12th-c church remains within the church structure itself, the earliest work dating from the 13th-c rebuilding. However, during the building work undertaken at the beginning of the 20thc, a number of carved Romanesque stones were found re-used in the foundations. These were photographed in place and a few have been preserved inside the church.

There is also inside the church the upper part of a medieval baptismal font, almost certainly carved in the 12thc. Some damage and evidence of weathering suggest it was outside for a period of time before being taken back into the church. The present shaft and base date from 1902.

Work was undertaken on the nave in 1806-7, which caused large changes to it, including the destruction of the transepts, while the choir was left in ruins. Then, in 1900-2 the choir was rebuilt, using as much of the surviving medieval stonework as possible, and restoration work was carried out on the other parts in an effort to make the church look more medieval.

The first mention of Brechin comes from a chronicle which records that Kenneth II, son of Malcolm, gave the ‘civitas’ of Brechin to the church in the late 10thc. Boece states that Brechin was destroyed by Danes about 1017 and that the round tower was all that remained from the church to the 'present day' (i.e. early 16thc). Culdee abbots are mentioned first in the margins of the Book of Deer, the first of these written additions dating from 1131-2. Leod, the earliest of the named abbots, seems to have played a prominent role in political affairs, appearing as a witness to numerous charters. The Culdees were to remain in Brechin until the early 13thc, the last abbot mentioned being Morgund. Thereafter, one hears only of bishops and canons.

King David seems to have been responsible for establishing the seat of a bishopric at Brechin about 1150 and the earliest named bishop, Samson, appears at about the same time. Initially established as an independent bishopric, the diocese of Brechin only became part of an archdiocese when St Andrews was raised to that status in 1472. The last bishop of the Brechin, Donald Campbell, died in 1562, but the bishopric continued after the Reformation until the time of Bishop James Drummond, who was deprived of his seat at the ‘Glorious Revolution’ of 1688-89. In 1638, the General Assembly of the Church of Scotland voted to expel bishops from the ‘kirk’, and their final, official expulsion came in about 1690.

Brechin was an unusual diocese in that its parishes were not compact but rather scattered. The exact number of parish churches within the early diocese has been disputed and remains unresolved, though it has been most recently argued to have been about 23, the majority of which were located in Angus. A Maison Dieu was founded in Brechin in 1256.

All, or part, of a church was built in the 12thc, as revealed by carved stones re-used in the foundations, but there is no documentary reference to its construction. In the early 13thc, work began on a new church building. Most of the surviving parts of the church date from this century, though other work was undertaken on parts of the structure in subsequent centuries.

There are no medieval documents existing that can substantiate a date for the construction of the round tower, which has led to a great deal of discussion, with suggestions ranging from the late-10thc to about 1100. Moreover, there is more than one 16th-c edition of Boece's Scotorum Historiae and more than one translation. They vary somewhat and only some mention the tower of Brechin's church. Brechin's tower is one of only two surviving round towers on the mainland of Scotland, the other being at Abernethy, with which it is often compared. MacGibbon and Ross state that “the first church in Brechin was founded by a colony of ecclesiastics, after the Irish model, about the beginning of the eleventh century, and the probability is that the tower was erected during that century.” Irish round towers of this type appear to date from the 10thc to the 13thc, but Fernie has argued that the tower at Abernethy is unlikely to date before the 1070s and is more likely to date to around 1100. Brechin, which compares favourably with Abernethy, he argues is of similar date. Cameron, however, states that the round tower of Brechin has two building phases, the earliest possibly late-10thc. Dates in the 11thc or early 12thc are most commonly given by other scholars.

Leod, abbot of Brechin, was at the dedication ceremony of Dunfermline Abbey in 1150. This may be relevant, as the surviving stones found in excavation, although harking back to work at Durham Cathedral, are most closely compared with carved stonework at Dunfermline Abbey Church. This suggests a date in the second half of the 12thc, with the likelihood of it being in the third quarter.

The baptismal font at Brechin is nearly identical to the one now in the parish church of Aberlemno (which was formerly at Aldbar Chapel, 2.4 miles SW of Brechin). It is almost certainly of 12thc date and most likely to be of similar date to the 12th-c work on the church.