similar. However, the cavalry of the Late Roman army was endowed with greater numbers of specialised units, such as extra-heavy shock cavalry (cataphractii and clibanarii) and mounted archers. During the later 4th century, the cavalry acquired a reputation for incompetence and cowardice for their role in three major battles. In contrast, the infantry retained its traditional reputation for excellence. The 3rd and 4th centuries saw the upgrading of many existing border forts to make them more defensible, as well as the construction of new forts with stronger defenses. The interpretation of this trend has fuelled an ongoing debate whether the army adopted a defence-in-depth strategy or continued the same posture of "forward defence" as in the early Principate. Many elements of the late army's defence posture were similar to those associated with forward defence, such as forward location of forts, frequent cross-border operations, and external buffer-zones of allied barbarian tribes. Whatever the defence strategy, it was apparently less successful in preventing barbarian incursions than in the 1st and 2nd centuries. This may have been due to heavier barbarian pressure, or to the practice of keeping large armies of the best troops in the interior, depriving the border forces of sufficient support. Sources Much of our evidence for 4th century army unit deployments is contained in a single document, the Notitia Dignitatum, compiled c. 395–420, a manual of all late Roman public offices, military and civil. The main deficiency with the Notitia is that it lacks any personnel figures, so as to render estimates of army size impossible. Also, it was compiled at the very end of the 4th century; it is thus difficult to reconstruct the position earlier. However, the Notitia remains the central source on the late Army's structure due to the dearth of other evidence. The Notitia also suffers from significant lacunae and numerous errors accumulated from centuries of copying. The main literary sources for the 4th-century army are the Res Gestae (History) of Ammianus Marcellinus, whose surviving books cover the period 353 to 378. Marcellinus, himself a veteran soldier, is regarded by scholars as a reliable and valuable source, but he largely fails to remedy the deficiencies of the Notitia as regards army and unit strength or units in existence, as he is rarely specific about either. The third major source for the late army is the corpus of imperial decrees published in the East Roman empire