least invest them, and requires for that, if the garrisons are brave and enterprising, perhaps double their strength. But, besides, these garrisons may and do mostly consist in part of troops, who, although competent to duty in a garrison, are not fit for the field -- half trained militia, invalids, convalescents, armed citizens, landsturm, etc. The enemy, therefore, in such case is perhaps weakened four times more than we are.
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8 As a protection to extended cantonments. That a moderate-sized fortress closes the approach to cantonments lying behind it for a width of fifteen to twenty miles is a simple result of its existence.
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9 As covering a province not occupied. If during war a province is either not occupied at all, or only occupied by an insufficient force, and likewise exposed more or less to incursions from flying columns, then a fortress, if not too unimportant in size, may be looked upon as a covering, or, if we prefer, as a security for this province. As a security it may at all events be regarded, for an enemy cannot become master of the province until he has taken it, and that gives us time to hasten to its defence. But the actual covering can certainly only be supposed very indirect, or as not properly belonging to it. That is, the fortress by its active opposition can only in some measure check the incursions of hostile bands.
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10 As the focus of a general arming of the nation. Provisions, arms, and munitions can never be supplied in a regular manner in a people’s war; on the other hand, it is just in the very nature of such a war to do the best we can; in that way a thousand small sources furnishing means of resistance are opened which otherwise might have remained unused; and it is easy to see that a strong commodious fortress, as a great magazine of these things, can well give to the whole defence more force and intensity, more cohesion, and greater results.

Besides, a fortress is a place of refuge for wounded, the seat of the civil functionaries, the treasury, the point of assembly for the greater enterprises, etc., etc.; lastly, a nucleus of resistance which during the siege places the enemy’s force in a condition which facilitates and favours the attacks of national levies acting in conjunction.

11 For the defence of rivers and mountains. Nowhere can a fortress answer so many purposes, undertake to play so many parts, as when it is situated on a great river. It secures the passage at any time at that spot, and hinders that of the enemy for several miles each way, it commands the use of the river for commercial purposes, receives all ships within its walls, blocks bridges and roads, and helps the indirect defence of the river, that is, the defence by a position on the enemy’s side. It is evident that, by its influence in so many ways, it very greatly facilitates the defence of the river, and may be regarded as an essential part of that defence.

Fortresses in mountains are important in a similar manner. They there form the knots of whole systems of roads, which have their commencement and termination at that spot; they thus command the whole country which is traversed by these roads, and they may be regarded as the true buttresses of the whole defensive system.