r it keeps sound and useful forever. And so not
only city walls but substructures in general and all walls that require
a thickness like that of a city wall, will be long in falling to decay
if tied in this manner.

4. The towers should be set at intervals of not more than a bowshot
apart, so that in case of an assault upon any one of them, the enemy may
be repulsed with scorpiones and other means of hurling missiles from the
towers to the right and left. Opposite the inner side of every tower the
wall should be interrupted for a space the width of the tower, and have
only a wooden flooring across, leading to the interior of the tower but
not firmly nailed. This is to be cut away by the defenders in case the
enemy gets possession of any portion of the wall; and if the work is
quickly done, the enemy will not be able to make his way to the other
towers and the rest of the wall unless he is ready to face a fall.

5. The towers themselves must be either round or polygonal. Square
towers are sooner shattered by military engines, for the battering rams
pound their angles to pieces; but in the case of round towers they can
do no harm, being engaged, as it were, in driving wedges to their
centre. The system of fortification by wall and towers may be made
safest by the addition of earthen ramparts, for neither rams, nor
mining, nor other engineering devices can do them any harm.

[Illustration: CONSTRUCTION OF CITY WALLS

(From the edition of Vitruvius by Fra Giocondo, Venice, 1511)]

6. The rampart form of defence, however, is not required in all places,
but only where outside the wall there is high ground from which an
assault on the fortifications may be made over a level space lying
between. In places of this kind we must first make very wide, deep
ditches; next sink foundations for a wall in the bed of the ditch and
build them thick enough to support an earth-work with ease.

7. Then within this substructure lay a second foundation, far enough
inside the first to leave ample room for cohorts in line of battle to
take position on the broad top of the rampart for its defence. Having
laid these two foundations at this distance from one another, build
cross walls between them, uniting the outer and inner foundation, in a
comb-like arrangement, set like the teeth of a saw. With this form of
construction, the enormous burden of earth will be distributed into
small bodies, and will not lie with all its weight in one crushing