While the Picts and the Scots of both Britain and Ireland lived in the relative security of remoteness, it was the misfortune of the Britons, abandoned by the imperial army whose protection they had enjoyed for centuries, to be left fully exposed to the attacks of the Anglo-Saxons. The military defences of Roman Britain were designed to restrain those of its people who lived in the less fertile mountainous districts and so to provide greater security for the development of agriculture, industry and the civilising influences of town life in the low-lying areas where the density of population was greater. Such a design presupposes that the North Sea and the English Channel were either free from shipping hostile to Britain or subject to the control of a Roman fleet. The building of fortifications and watch-towers along parts of Britain's eastern and southern coasts marks the beginning of a process which was spread over some three centuries and which resulted in the occupation of the richer lowland areas by the English and the confinement of the Britons who remained independent to the remoter hills. The seizure of the richer areas by pagan barbarians cannot but have seemed a total disaster to those who were dispossessed, people who certainly by the year 600, and many of them one or even two centuries earlier, were members of a literate Christian society familiar with books and even the niceties of theological argument.