Fortresses should always occupy the important strategic points
already designated in Article XIX. As to their tactical qualities, their
sites should not be commanded, and egress from them should be easy, in
order to increase the difficulty of blockading them.
Those which possess the greatest advantages, either as to their own
defense or for seconding the operations of an army, are certainly those
situated on great rivers and commanding both banks. Mayence, Coblentz,
and Strasbourg, including Kehl, are true illustrations and models of
this kind. Places situated at the confluence of two great rivers command
three different fronts, and hence are of increased importance. Take, for
instance, Modlin. Mayence, when it had on the left bank of the Main the
fort of Gustavusburg, and Cassel on the right, was the most formidable
place in Europe, but it required a garrison of twenty-five thousand men:
so that works of this extent must be few in number.
Large forts, when encompassing populous and commercial cities, are
preferable to small ones,—particularly when the assistance of the
citizens can be relied on for their defense. Metz arrested the whole
power of Charles V, and Lille for a whole year delayed Eugene and
Marlborough. Strasbourg has many times proved the security of French
armies. During the last wars these places were passed without being
besieged by the invading forces, because all Europe was in arms against
France; but one hundred and fifty thousand Germans having in their front
one hundred thousand French could not penetrate to the Seine with
impunity, leaving behind them these well-fortified points.
Formerly the operations of war were directed against towns, camps, and positions; recently they have been directed