Gun-Embrasures and Defense of Mine-Galleries
Square vented embrasures between towers, and lowering defenders into fortress mines by winch
Folio 91r continues Leonardo's treatise on fortification. The main note prescribes square gun-embrasures set in the walls between a fortress's flanking towers, made wide enough to vent the flame of the bombards so it does not blow the mouth of the embrasure to the ground or reflect back and suffocate the gunners, and warns against machicolations on walls that flanking towers already defend, since the enemy tears them down at the first assault. A further passage insists that underground mine-galleries be entered only by the most difficult ways, lowering defenders in a box on a winch as one lowers a bucket down a well, because fortresses are most often taken through such tunnels. A cross-section of an embrasure (labelled a) and a small fortress plan accompany the text, and faint offset writing shows through in the left margin.
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Vented square embrasures between the towers
In the wall-spaces between the towers a gun-embrasure is set eight feet square, with a large iron-barred window opened as needed and its entrance from the flank, below the level of the fortress floor. The great width is needed to vent the flame of the bombards, for a cramped mouth would throw its own lips to the ground or reflect the flame back and choke the gunners. A mantlet of beams shields the instruments.
No machicolations on flank-defended walls
Machicolations should not be built on fortress walls that the adjoining towers can already defend from the flank. The enemy throws them down at the first assault, leaving the walls stripped of defense, so that men who were to reach the towers by that route can no longer move without being seen and shot.
Winch and box for descending into the mines
Defenders should be sent down into the fortress's underground caves not by easy ways but by the most difficult, lowered in a box raised and lowered by a winch at the castellan's pleasure, in the manner of a bucket down a well or cistern. Leonardo notes this precaution because fortresses are most often taken through such underground galleries.
Cross-section of an embrasure (a)
At the lower left a cutaway of a wall shows the flanked, splayed opening of a gun-embrasure, keyed with the letter a. A caption beside it notes that the arrangement is 'of little expense'.