Boreum: name of a Byzantine fortress in modern Libya, now called Bu Grada.
In the fifth century, the new tribal federation of the Laguatan threatened Roman Cyrenaica. Texts like Epistle 73 by Synesius of Cyrene, written in 409, describe the problems of the inhabitants, who felt abandoned by the central government. It is not entirely clear how far the invaders actually got, but the crisis was severe. Cyrene, for instance, was abandoned.
More than a century later, in the 530s, the Byzantine emperor Justinian (527-565), sent garrisons to Paraetonium (modern Marsa Matruh) and Antipyrgon (Tobruk).This reorganization is known as the Ananeosis. He also fortified Ptolemais, Taucheira/Arsinoe, Berenice/Euesperides (modern Benghazi), two monasteries, and a place called Boreion or Boreum, which has been identified by R.G. Goodchild with a promontory northeast of modern Marsa al-Brayqah.
The town is also mentioned by Ptolemy of Alexandrianote[Ptolemy, Geography 4.4.] and described by Procopius:
Here the mountains press close upon one another, and thus forming a barrier by their crowding, effectively close the entrance to the enemy. This city, which had been without a wall, the Emperor enclosed with very strong defenses, thus making it as safe as possible for the future, together with the whole country round about it.note[Procopius, Buildings 6.2.12-13.]
The remark about the mountains betrays that Procopius never visited Boreum, because the country is actually pretty flat. Between two harbors, several remains are clearly visible, like a wall with a moat, a pier, a tunnel from the citadel to the eastern harbor, rock chambers, and a large structure that has been called "bastion", although it is not known what it really is.
The site, which has not been excavated, is locally known as "Castle of Mary". It is 12¾ km Northeast of Marsa al-Brayqah; you must leave the main road about 8½ km east of this city, turn to the left, almost immediately turn to the right, continue to the North (later Northwest) for almost 4 km and, once you have reached the coast, turn right and continue to the Northeast for 4½ km. Visiting the site is not without danger, as there are several ancient cisterns hidden under the dunes.
- Erwin Ruprechtsberger, Die römische Limeszone in Tripolitanien und der Kyrenaika, Tunesien - Libyen (1993 Aalen; Limes Museum).