For mountainous frontiers it is deemed necessary to secure all the
important passes with small redoubts or military works, and to defend
with strong forts the grand interior strategic points on which these
communications are directed. For a frontier of moderate extent there may
be some six or eight gorges in the mountains by which an army might
penetrate; but it will always be found that these roads concentrate on
two or three points in the great valleys below. Take, for example, the
frontier of France towards Switzerland and Italy. The passes of the
mountains are secured by the little works of Fort L'Ecluse, Fort
Pierre-châtel, Fort Barraux, Briançon, Mont Dauphin, Colmars, Entrevaux,
and Antibes; while Besançon, Grenoble, and Toulon, form a second line;
and Lyons a grand central dépôt.

Where a great river or chain of lakes forms the boundary of a state, the
system of defence will be much the same as that of an open land
frontier, the works of the first line being made to secure the great
bridges or ferries by which the enemy might effect a passage; those of
the second line, to cover the passes of the highlands that generally
approach more or less near the great watercourse; and those of the third
line, far enough in rear to protect the great internal communications of
the country. Let us take, for example, the side of France bordering on
the Rhine. Wissembourg and Lauterbourg, Fort Louis, Haguenau,
Strasbourg, Schelstadt, Neuf-Brisach, and Huneguen, cover the several
passages of the river; while Bitche, Phalsbourg, and Béfort form a
second line; Thionville, Metz, and Toul, a third line; and Verdun a
grand central dépôt.

The following are the principal objects proposed to be accomplished by
fortifications on a sea-coast.