Fortresses Adapted to Their Sites
Angular plans for a town in a valley and a fortress open on one side; mutually flanking ravelins
Folio 93r opens with the principle that fortresses must be varied to suit their sites according to their nature. Leonardo works through cases: a town straddling two hills with a valley between should follow the slopes in an acute-angled plan, with little towers at its tips and triangular flanking gates at each mouth of the valley, while a fortress open to attack from only one side gets a solid acute angle 25 to 30 feet across with lateral defenses, gates on those flanks and bastions and retreats behind them. Marginal notes explain that at these points fascines and mantlets would only be burned, and that the gun-embrasures of each ravelin sweep the flanks of the next so that no ladders can be planted. Bird's-eye and profile plans of hillside fortresses, one captioned bridge, illustrate the cases.
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Fortresses varied to their sites
The opening maxim holds that the various sites require the fortresses to be varied according to their nature, framing the case studies that follow as adaptations of one system to different terrain.
A town between two hills over a valley
Where two hills flank a valley, the town extends along both slopes in the form of an acute angle, with little towers at its farthest extremities. At each mouth of the valley stand gates shaped like a triangle, with offensive positions and lateral entrances furnished with ditches and ravelins.
A fortress open to attack on one side only
If the place can be assailed from a single side, that side is given a solid acute angle 25 to 30 feet across with lateral defenses. The gates are set on those flanks, with bastions behind them and suitable retreats.
Ravelins that flank one another
The gun-embrasures of one ravelin see and sweep the sides of the next, and so each does to another in turn, so that no ladders can be planted there. Elsewhere Leonardo notes that fascines set at these points would be burned and mantlets could not be brought up to cut the walls.