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