A document now known as the "Burghal Hidage" provides an insight into how the system worked. It lists the hidage for each of the fortified towns contained in the document. Wallingford had a hidage of 2,400, which meant that the landowners there were responsible for supplying and feeding 2,400 men, the number sufficient for maintaining of wall. A total of 27,071 soldiers were needed, approximately one in four of all the free men in Wessex. Many of the burhs were twin towns that straddled a river and were connected by a fortified bridge, like those built by Charles the Bald a generation before. The double-burh blocked passage on the river, forcing Viking ships to navigate under a garrisoned bridge lined with men armed with stones, spears or arrows. Other burhs were sited near fortified royal villas, allowing the king better control over his strongholds.