Machiavelli discusses a manner in which the strength of a principality may be measured. Machiavelli draws a distinction between princes who possess territory and resources such that they can "stand alone" and those who "must always have recourse to the protection of others." Machiavelli defines the latter group of princes as those who "cannot take the field against the enemy but are forced to retreat behind walls and make their defense there." In this case, Machiavelli advises these princes to fortify their towns and "not to worry" about the surrounding countryside. If a prince has fortified his towns well, then enemies will think twice before launching an attack against him. Machiavelli states, "It is obviously not easy to assault a town which has been made into a bastion by a prince who is not hated by the people."
Machiavelli describes two powerful ways in which a prince may secure his status and his state: through the use of arms and fortifications and through the goodwill of the people. Princes who possess both physical defenses and popular support protect themselves with highly effective and complementary means and thus make unappealing targets for foreign aggressors. With walls and forts a prince wards off external threats; with the support of his subjects a prince guards himself against internal disorder and opposition.
Citing the modern examples of independent German cities, Machiavelli writes, "A prince who has a well-fortified city and does not make himself hated is secure against attack." Because the German cities are protected with moats, walls, and "public stocks of drink, food, and fuel" for their people, would-be attackers avoid these cities, knowing that any siege would be "protracted" and difficult. If a siege does in fact occur, Machiavelli argues that a "powerful, courageous prince" will always overcome the ensuing difficulties, inspiring his subjects with hope and spreading "fear of the enemy's cruelty." Machiavelli even states that the burning and pillaging of the countryside by an enemy may unite the people and prince in shared hatred of the attacker. In short, a prudent prince, with sufficient fortifications and provisions, may easily overcome a siege.
According to Machiavelli, a prince who finds himself under siege must draw on his prowess to unite the people in fear and hatred of the aggressor. A prince must work to direct the people's anger and frustration at the foreign foe and unite his state in hatred of a common enemy. The goodwill of the people serves as a central pillar of a prince's defense, although the prince must possess the prowess necessary to unify his people in difficult times. To overcome a siege, a prince requires the support of the people as much as he needs physical fortifications and supplies.