The wall extended across the island, from the mouth of the Tyne, on the German Ocean, to the Solway Frith--nearly seventy miles. It was twelve feet high, and eight feet wide. It was faced with substantial masonry on both sides, the intermediate space being likewise filled in with stone. When it crossed bays or morasses, piles were driven to serve as a foundation. Of course, such a wall as this, by itself, would be no defense. It was to be garrisoned by soldiers, being intended, in fact, only as a means to enable a smaller number of troops than would otherwise be necessary to guard the line. For these soldiers there were built great fortresses at intervals along the wall, wherever a situation was found favorable for such structures. These were called _stations_. The stations were occupied by garrisons of troops, and small towns of artificers and laborers soon sprung up around them. Between the stations, at smaller intervals, were other smaller fortresses called castles, intended as places of defense, and rallying points in case of an attack, but not for garrisons of any considerable number of men. Then, between the castles, at smaller intervals still, were turrets, used as watch-towers and posts for sentinels. Thus the whole line of the wall was every where defended by armed men. The whole number thus employed in the defense of this extraordinary rampart was said to be ten thousand. There was a broad, deep, and continuous ditch on the northern side of the wall, to make the impediment still greater for the enemy, and a spacious and well-constructed military road on the southern side, on which troops, stores, wagons, and baggage of every kind could be readily transported along the line, from one end to the other. [Illustration: WALL OF SEVERUS] The wall was a good defense as long as Roman soldiers remained to guard it. But in process of time--about two centuries after Severus's day--the Roman empire itself began to decline, even in the very seat and center of its power; and then, to preserve their own capital from destruction, the government were obliged to call their distant armies home. The wall was left to the Britons; but they could not defend it. The Picts and Scots, finding out the change, renewed their assaults. They battered down the castles; they made breaches here and there in the wall; they built vessels, and, passing round by sea across the mouth of the Solway Frith and of the River Tyne, they renewed their old incursions for plunder and destruction. The Britons, in extreme distress, sent again and again to recall the Romans to their aid, and they did, in fact, receive from them some occasional and temporary succor. At length, however, all hope of help from this quarter failed, and the Britons, finding their condition desperate, were compelled to resort to a desperate remedy, the nature of which will be explained in the next chapter.