CASTLE, or Castle-Steed, is also an appellation given by the country people in the north to the Roman castella, as distinguished from the castra stativa, which they usually call cheesters. Horsley represents this as a useful criterion, by which to discover or distinguish a Roman camp or station. There are several of these castella on Severus's wall. They are generally sixty feet square; their northern side is formed by the wall itself, which falls in with them; the intervals between them are from six furlongs and a
half to seven; and they seem to have stood closest where the stations are widest. The neighbouring people call them castles, or castle-steeds, by which it seems probable that their ancient Latin name had been castellum. Some modern writers call them mile castles, or military castella; Horsley sometimes denominates them exploratory castles. In these castella were stationed men whose business was to make incursions into the enemy's country and give intelligence of their motions.