Chapter 20 — Are Fortresses and Other Defenses Advantageous or Harmful?

The chapter opens with a typology of defensive strategies that different princes have employed. Some have disarmed their subjects, fearing that an armed population could turn against them; others have fostered divisions between factions within their cities to prevent any unified opposition from forming; others have cultivated enemies at the start of their reign in order to demonstrate their own strength in overcoming them; others have built physical fortresses as refuges against sudden attack or rebellion. Machiavelli examines each strategy against the historical record.

Disarming subjects is counterproductive for a new prince: it signals distrust and creates exactly the resentment it was designed to prevent. Those who were disarmed feel insulted; they are no longer loyal. Those who were already armed feel that their weapons were purchased at the expense of others' disarmament. An armed citizenry, properly managed, becomes an extension of the prince's power rather than a threat to it. Dividing a city into factions is similarly mistaken: it may have worked when the Italian states were in balance, but the moment a foreign power enters, divided cities are the easiest to take — each faction seeks foreign support against the other and delivers the city to the outsider.

Fortresses receive the most nuanced treatment. They are not inherently wrong — they have been useful in specific historical circumstances — but they cannot substitute for the goodwill of the population. Machiavelli cites Catherine Sforza's defence of her Forlì fortress after her husband's assassination as a case where the fortress preserved her position long enough for a relief force to arrive. But when Cesare Borgia attacked with popular support behind him, the fortress could not save her. The principle that emerges is consistent with the rest of the book: a prince who is hated by his people has no effective defence regardless of his physical fortifications, because his enemies will always find allies within his own walls.