much larger than new-build. New forts were rarely over one hectare in size and were normally placed to fill gaps between old forts and towns. However, not all of the old forts that continued to be used in the 4th century were upgraded e.g. the forts on Hadrian's Wall and some other forts in Britannia were not significantly modified. The main features of late Roman fortification clearly presage those of medieval castles. But the defensibility of late Roman forts must not be exaggerated. Late Roman forts were not always located on defensible sites, such as hilltops and they were not designed as independent logistic facilities where the garrison could survive for years on internal supplies (water in cisterns or from wells and stored food). They remained bases for troops that would sally out and engage the enemy in the field. Nevertheless, the benefits of more defensible forts are evident: they could act as temporary refuges for overwhelmed local troops during barbarian incursions, while they waited for reinforcements. The forts were difficult for the barbarians to take by assault, as they generally lacked the necessary equipment. The forts could store sufficient supplies to enable the defenders to hold out for a few weeks, and to supply relieving troops. They could also act as bases from which defenders could make sorties against isolated groups of barbarians and to cooperate with relieving forces. The question arises as to why the 4th-century army needed forts with enhanced defensive features whereas the 2nd-century army apparently did not. Luttwak argues that defensible forts were an integral feature of a 4th-century defence-in-depth "grand strategy", while in the 2nd century "preclusive defence" rendered such forts unnecessary . But the existence of such a "strategy" is strongly disputed by several scholars, as many elements of the late Roman army's posture were consistent with continued forward defence. An alternative explanation is that preclusive defence was still in effect but was not working as well as previously and barbarian raids were penetrating the empire more frequently.(see Strategy, below) Strategy and tactics Strategy Edward Luttwak's Grand Strategy of the Roman Empire (1976) re-launched the thesis of Theodor Mommsen that in the 3rd and early 4th centuries, the empire's defence strategy mutated from "forward defence" (or "preclusive defence") in the Principate to "defence-in-depth" in the 4th century. According to Luttwak, the army of the Principate had relied on neutralising imminent barbarian incursions before