In the case of the Vimianzo Castle, the keep is not the largest tower, because this merit belongs to the east-facing tower, nor is it the first, since potential pillars of another keep were discovered in the middle of the courtyard. However, the keep is the tallest and retains its full battlements: its brackets with six moldings, its machicolation adorned with blackbirds, merlons, and arrow slits. It was the last defensive resort to garrison against enemy attacks, as evident in the small holes of different windows (flared and arrow-slitted) which scarcely allow light into the interior of the building so as to not detract from the wall strength, except for the arced window at the top of the north face, which dates from modern times. In addition, the tower had a complicated entrance; it could only be accessed by a wooden staircase that reached the corbel-arched door on the north side.
Among the various bellicose episodes having taken place in this fortress, perhaps the best known is the attack of the Irmandiños from Terra de Soneira in 1468 which ended with its partial destruction. Another intense attack was that led by the warden Xoán Mariño de Lobeira, by order of Archbishop Alonso II of Fonseca, at the helm of forty men, to unsuccessfully defend against Lopo Sánchez de Moscoso in 1472. From this moment, until 1870 (nearly four hundred years), Vimianzo Castle would be associated with the lineage of Moscoso, the powerful Counts of Altamira.