Where to site a fortress, and defending its enclosures
Fortresses low for their moats or high on nearby hills, with a circular fortress plan
Four notes on fortress design surround studies of towers, buildings, a spiralled form and, at lower right, a circular fortress plan marked 30, 20, 30, 20, 30, 20, 40. Leonardo advises siting fortresses on the lowest level ground so the water of their ditches is not lost, but upon nearby hills wherever any exist so they cannot be bombarded from above. He warns that a relief force must not enter the castellan's keep and grow stronger than he, as happened at Fossombrone, and that each successive enclosure be defended from the one before it; a brief note calls for pillars within the bastions.
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Siting a fortress for its ditches or on the hills
Fortresses ought to stand on the lowest part of level ground, so that the water of their ditches is not taken from them. But if hills or mountains lie nearby, the fortresses should instead stand upon those hills, so they cannot be battered from above.
Relief must not enter the keep — Fossombrone
The relief force must not enter the keep of the castellan, lest it become more powerful than he, as happened at Fossombrone. The covering of each succeeding enclosure must be defended from the one preceding it.
Pillars inside the bastions
A short note in the lower left calls for pillars to be set within the bastions, supporting their structure.
Plan of a circular fortress
At the lower right a circular fortress is laid out in plan, its segments carrying the dimensions 30, 20, 30, 20, 30, 20 and 40. Concentric rings and radial marks set out its defensive rings.