sentences:  Along with a continuous wall (except in the case of Gask Ridge), there existed a metaled road immediately behind the wall for transport of troops.Along the wall there existed a few large forts for legions or vexillations, as well as a series of milecastles - effectively watchtowers that were unable to defend a stretch of wall against anything but low-scale raiding but were able to signal attack to legionary forts by means of fire signals atop the towers.In the later Empire, Roman Britannia found itself increasingly vulnerable to external aggression, in parallel to attacks felt across the length of the Empire's borders.