- As real barriers, they close the roads, and in most cases the rivers, on which they are situated.
It is not as easy as is generally supposed to find a practicable lateral road which passes round a fortress, for this turning must be made, not only out of reach of the guns of this place, but also by a detour greater or less, to avoid sorties of the garrison.
If the country is in the least degree difficult, there are often delays connected with the slightest deviation of the road which may cause the loss of a whole day’s march, and, if the road is much used, may become of great importance.
How they may have an influence on enterprises by closing the navigation of a river is clear in itself.
- As tactical points d’appui. As the diameter of the zone covered by the fire of even a very inferior class of fortifications is usually some leagues, fortresses may be considered always as the best points d’appui for the flanks of a position. A lake of several miles long is certainly an excellent support for the wing of an army, and yet a fortress of moderate size is better. The flank does not require to rest close upon it, as the assailant, for the sake of his retreat, would not throw himself between our flank and that obstacle.
As a station ( or stage ). If fortresses are on the line of communication of the defensive, as is generally the case, they serve as halting places for all that passes up and down these lines. The chief danger to lines of communication is from irregular bands, whose action is always of the nature of a shock. If a valuable convoy, on the approach of such a comet, can reach a fortress by hastening the march or quickly turning, it is saved, and may wait there till the danger is past. Further, all troops marching to or from the army, after halting here for a a few days, are better