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.
[Footnote 1: For some account of the circumstances connected with this war see our history of Alexander, chapter vi.].