In addition to the Stations, CASTELLA or MILE-CASTLES were provided for the use of the troops which garrisoned the Wall. They derive their modern name from the circumstance of their being usually placed at the distance of a Roman mile from each other. They were quadrangular buildings, differing somewhat in size, but usually measuring from sixty to seventy feet in each direction. With two exceptions, they have been placed against the southern face of the Wall; the castle at Portgate, every trace of which is now obliterated, and another near ÆSICA, the foundations of which may, with some difficulty, still be traced, seem to have projected equally to the north and south of the Wall. Though generally placed about seven furlongs from each other, the nature of the ground, independently of distance, has frequently determined the spot of their location. Whenever the Wall has had occasion to traverse a river or a mountain pass, a mile-castle has uniformly been placed on the one side or other to guard the defile. The mile-towers have generally had but one gate of entrance, which was of very substantial masonry, and was uniformly placed in the centre of the south wall; the most perfect specimen now remaining, however, has a northern, as well as a southern gateway. It is not easy to conjecture what were the internal arrangements of these buildings; probably they afforded little accommodation beyond what their four strong walls and well-barred gates gave. Hodgson states that when the foundations of the castle northeast of Housesteads were removed in 1832, the remains of an inner wall were seen, all round, parallel to the outer walls. He hence infers that the space between the walls has been roofed, and the centre uncovered. Deferring the further discussion of this subject until, in the course of our local description, we arrive at the most perfect specimen remaining—the mile-castle near Cawfields—the reader is meanwhile referred to the lithograph which depicts this interesting remain. [Sidenote: THE TURRETS OR WATCH TOWERS.]