Though no part of the wall now retains its full height, it has been calculated that when entire it was about eighteen or nineteen feet, including the parapet. Its general thickness is about eight feet, though it varies in different parts from six feet to ten and a half feet. It is "throughout the whole of its length accompanied on its northern margin by a broad and deep fosse, which, by increasing the comparative height of the wall, added greatly to its strength. This portion of the barrier may yet be traced, with trifling interruptions, from sea to sea." [Illustration: LONGITUDINAL SECTION OF THE ROMAN WALL.] The masonry of the wall is somewhat peculiar; it has none of the binding courses of tile which are, in many parts of England and on the Continent, so characteristic of Roman work. No tiles are used in the construction. The outer face of the wall on both sides is formed of squared blocks of stone, usually called ashlar, and the interior of rubble, embedded in mortar. These blocks are about eight or nine inches thick, and ten or eleven wide; their length is considerably more, sometimes as much as twenty-two inches, and tapering to the opposite end, which was firmly bedded into the rubble. The whole rested on a course of large foundation stones. On or near the wall were placed, at tolerably regular intervals, stationary camps, or "stations," about seventeen or eighteen in number; and at still shorter intervals, that is, about a Roman mile from each other, were placed smaller towers, called, from this circumstance, "mile castles." These are in general placed against the south side of the wall, and had mostly only one entrance, which was from the south; but in the most perfect of those at Gawfields there are two entrances, one on the south, and another through the main wall on the north. [Illustration: CORNICE FROM VENDALANA (CHESTERHOLM).]