The burhs were connected by a road system maintained for army use (known as herepaths). The roads allowed an army quickly to be assembled, sometimes from more than one burh, to confront the Viking invader. The road network posed significant obstacles to Viking invaders, especially those laden with booty. The system threatened Viking routes and communications making it far more dangerous for them. The Vikings lacked the equipment for a siege against a burh and a developed doctrine of siegecraft, having tailored their methods of fighting to rapid strikes and unimpeded retreats to well-defended fortifications. The only means left to them was to starve the burh into submission but this gave the king time to send his field army or garrisons from neighbouring burhs along the army roads. In such cases, the Vikings were extremely vulnerable to pursuit by the king's joint military forces. Alfred's burh system posed such a formidable challenge against Viking attack that when the Vikings returned in 892 and stormed a half-built, poorly garrisoned fortress up the Lympne estuary in Kent, the Anglo-Saxons were able to limit their penetration to the outer frontiers of Wessex and Mercia. Alfred's burghal system was revolutionary in its strategic conception and potentially expensive in its execution. His contemporary biographer Asser wrote that many nobles balked at the demands placed upon them even though they were for "the common needs of the kingdom".