sentences:  This left them with a border shared with a people who made repeated raids and insurrections against them.Unlike other borders throughout the empire, there was no natural border to fall back on such as desert or wide river that crossed the whole peninsula, so instead a series of defenses were built in southern to mid-Scotland in order to protect the province of "Britannia" from the Caledonians and later the Picts.Although the border was not a continuous wall, a series of fortifications known as Gask Ridge in mid-Scotland may well be Rome's earliest fortified land frontier.