<P> In the south of Mauritania Tingitana Romans made a limes in the third century, just north of the area of actual Casablanca near Sala and stretching to Volubilis . </P> <P> Septimius Severus expanded the "Limes Tripolitanus" dramatically, even briefly holding a military presence in the Garamantian capital Garama in 203 AD . Much of the initial campaigning success was achieved by the legate of Legio III Augusta, Quintus Anicius Faustus . </P> <P> Following his African conquests, the Roman Empire may have reached its greatest extent during the reign of Septimius Severus, under whom the empire encompassed an area of 2 million square miles (5.18 million square kilometers). </P>

What three features formed the central northern boundary of the roman empire