Citadels are small fortresses, usually of 4 to 5 fronts. They are raised in fortified towns to secure the possession of them, in case the inhabitants revolt; and to furnish the garrison a retreat, if threatened by them, from which they may punish their rebellion. They also receive the remains of the garrison when the town is taken, and compel the enemy to undertake a second siege. They thus furnish the means of reoccupying the town if assistance arrives.
The citadel should be large enough to accomodate the remains of the garrison. This may be estimated at three fourths of the original number, of which one fourth are probably in the hospital. It should be provided with bomb-proof lodgments sufficient to contain every thing necessary to enable this garrison to sustain as long a siege as the strength of the fortification will admit. (Bousmard, vol. 3)
When the citadel is adjacent to the fortification of the town, its covered way should be separated from the houses by a large esplanade, so that they may not favour the enterprise of the enemy. When the citadel is without the town, it is connected with it by lines of communication, and the fortification of the citadel should possess an influence over that of the town.
In great fortified towns, which have no citadel to oppose rebellion, or awe the inhabitants, one or two bastions are entrenched at the gorge, and thus become reducts for the garrison, and fortifications against the people.
Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)