 Fortifications were also constructed at transport choke points, to collect tolls and to restrict military passage; fortified towns grew up alongside all bridges and most fords over the many rivers in the region.Military forces could support themselves by foraging so long as they moved on at frequent intervals.If they wished to remain in one place for any length of time, as was necessary to besiege a castle, then access to water transport was essential for supplies of food and fodder and desirable for such items as siege equipment.