So long as the custody of the castle and that of the borough were closely associated, as most certainly was the case during the fourteenth and fifteenth centuries, the whole responsibility of maintenance rested with the Crown. The burgesses made the most of their political status as a useful means of soliciting aid from the Crown. The walls of Carnarvon towards the castle on the west side were erected by royal command in 1326, and the western gate of the town, unfortunately burnt in the reign of Edward I., was similarly repaired. The alleged purpose of these new works was the better keeping of the castle and town. (fn. Cal. Close Rolls, 1323-7, pp. 445, 451) Seven years previously the burgesses of Carnarvon had petitioned the Crown to repair the bridge of the 'great gate' leading into their town. (fn. Cal. Close Rolls, 1318-23 p. 165) The request was fulfilled two years later, after the King was given to understand that he had been wont to repair the bridge. (fn. Cal. Close Rolls, 1318-23 p. 406) Even as late as the Tudor period (fn. Min. Acc., 23 Henry VII., No. 1600) English sovereigns spent money on this same bridge. The liability of defence at Carnarvon, as in the other castle boroughs, is very typical. It was the King's planks, hewed at public expense in the royal forest of Snowdon, that bridged the Seiont there, and the King's gates leading through the town walls were continually guarded by experienced watchmen at the royal expense. (Lewis)
Clearly here the importance was access to the town, not defence, but the bridge is described as part of the defenses.