CASTLE, or Castle-stead, 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 cheisters. Horsley represents this as a useful criterion, whereby to discover or distinguish a Roman camp or station. There are several of these castella on Severus's wall: they are generally 60 feet square; their north side is formed by the wall itself which falls in with them; the intervals between them are from six furlongs and an half to seven; they seem to have stood closest where the stations are widest. The neighbouring people call them castles or castle-steads, by which it seems probable that their ancient Latin name had been castellum. Some modern writers call them mid-castles, or military castella: Horsley sometimes exploratory castles. In these castella the aurum had their stations, who were an order of men whose business was to make incursions into the enemies country, and give intelligence of their motions.