Wagons like carriages are a horse drawn vehicles.
In Daventry 'wagon length' is a way of measuring distance and space. For example the massive iron doors leading into Sorrowing Court was wide enough to allow three wagons side by side to enter, and tall enough to pull in a house. Karn Megiddo also later knocks Dunstant a 'wagon length' away.
Castle Daventry has several wagons and a royal carriage, which are drawn by the horses of the castle's stable. They have a large hay wagon which was pulled by a two horse team for bring hay to the castle to feed the horses in the stable.