A prince who fears his subjects ought to build fortresses in his country to hold them in obedience. (Discourses, book 2 chapter 24, and Prince chapter 20)
The prince who has more fear of his own people than of foreigners must build fortresses; but he who doubts foreigners more than his subjects need not. For the best fortress that is, is not to be thought evil by subjects; and if a prince is once thought so, there is no fortress that can save him. It is true that fortresses may be profitable to a prince in time of peace, to give more courage to him and to his governors established in them, to hold the people in subjection, and to use against them greater audacity and rigor. But yet this shall be but weak assurance, unless the prince has means to raise up a good and strong army to tame his subjects if they rebel. For to think to tame them by reducing them to poverty, Spoliatis arma supersunt: arms remain yet to the unarmed. Also to disarm them, Furor arma ministrat: fury administers arms enough. Likewise, to slay the chief heads of the people, more heads would arise, as of the Hydra. The Sforces built the castle at Milan and judged that by the means of that fortress they might with assurance handle their subjects at their pleasure, and therefore spared no kind of violence. Thus they acquired the hatred and evil will of their subjects, which was the cause that the French, their enemies, carried away Milan at the first assault; and the Sforces had no good by their fortress, but were spoiled of all the duchy.
Although Machiavelli has not dealt with the art of tyranny in his writing by a method, yet he has not left behind any part of that art. For first he has handled how a tyranny ought to be built; that is, by cruelty, perfidy, craft, perjury, impiety, revenges, contempt of counsel and friends, entertainment of flatterers, trumpery, hatred of virtue, covetousness, inconstancy, and other like vices; whereby he has demonstrated that men must ascend by degrees to come unto a sovereign wickedness. Secondly, he has showed how one ought to be maintained and conserved in that high degree of wickedness and tyranny; namely, by maintaining among subjects partiality and seditions, and in holding them in poverty and necessity. Now he adds another means, namely, to build fortresses against his subjects, as by making citadels in good towns, and by building forts upon bridges and common passages, and other like castles and fortresses. And Machiavelli thinks this means ought to be practiced, and that other aforesaid means are not sufficient to establish a tyranny. For poverty, he says, is no sufficient means to contain a people in obedience, for they are never without arms. And though they should disarm them and slay their chieftains, yet that would not suffice, because the anger and fury of the people would furnish them with sufficient arms, and chieftains would arise unto them like the heads of a Hydra.
But I will not stay long in the refutation of this maxim; I will only say this, that experience makes us wise, and that the invention of the citadels which in our time princes have built against their subjects, has been the cause of infinite evils. For all commerce and traffic has been and is greatly diminished in towns where they have been built, and there have been and are committed infinite insolences by soldiers against citizens; and no good has come to their princes but great expenses and the evil will of their subjects. For this construction of citadels is an apparent show that the prince does not trust his subjects, especially when they are built anywhere but at the borders of countries. When the subjects know that their prince distrusts them, they also esteem that he does not love them. And when the subject is not beloved by his prince, he cannot also love him; and not loving him, he obeys him not but as constrained, and in the end will get his head out of the yoke, as soon as there falls out a fit occasion. Here is the profit of citadels.
Yet I will say this by the way, that our Machiavellians of France, who were authors and enterprisers of the massacres of Saint Bartholomew, read not well this place in Machiavelli. For they said that men must not stay upon fishing for frogs, but must catch in their nets great salmons; and that one salmon’s head was worth more than ten thousand frogs, and when they had slain the chieftains of pretended rebels, they should easily overthrow the rude and rascally multitude, which without head could enterprise nothing. These venerable enterprisers should have considered what their doctor Machiavelli says here, which they have since seen by experience: that a people cannot want heads, which will always rise up, even those heads which are slain. If they had so well noted and practiced this point of Machiavelli as they do others, so much blood would never have been shed, and their tyranny may have longer endured. For the great effusion of blood immediately cried for vengeance to God, who according to his accustomed justice has heard the voice of that blood; and for the cry of the orphan and the widow, he has laid the axe to the root of all tyranny, and already has cut away many branches thereof; and if it please him will not tarry long to lay all on the ground, and so establish France in its ancient government.
As for fortresses in frontiers of countries, they have long been practiced, and are profitable to guard from incursions and invasions of enemies, that those who dwell near borders may the more peaceably enjoy their goods. We read that the emperor Alexander Severus gave his frontier fortresses to good and approved captains to enjoy during their lives, to the end (according to Lampridius) that they might be more vigilant and careful to defend their own. And afterward the emperor Constantine ordained that the said fortresses, with their grounds and revenues, should pass to the heirs of the said captains, who held them as a heritage. And hereupon, some say, has come the civil law called Feudi.