Large armed forces needed transport systems with enough capacity to sustain them. Even Rome's road network did not remove the value of its navigable rivers for moving goods and military supplies. Forts along a river could use it as both a natural defence and a route for resupply. A siege normally depends on the attacker retaining access to supplies while denying them to the garrison. Continued river access could frustrate that attempt to starve a fortress into submission.
Notable Point · 9:43-11:33
Roman frontier forts depended on river resupply
Roman rivers combined a defensive boundary with a supply route that made frontier garrisons harder to besiege.