Outworks and Mortar-Proof Towers
Corner entrances with triangular works and ravelins; a tower roofed obliquely to deflect mortar fire
Folio 92v sets out Leonardo's designs for outworks and mortar-proof towers. He specifies corner entrances screened by paired parallel walls and triangular forward works leading to a bastion with gates, ravelins and ditches, and prescribes that a square fortress vulnerable at two corners be given solid corners with sheds above and flanking towers with lateral offenses. Against mortar fire he designs a tower whose scarp converges to a point at its summit and whose corbelled corridor, divided into as many rooms as it has crossbow-loops, is roofed with an oblique vault so that shots glance off, its walls lined inside with bound oak. Bird's-eye fortress plans, two tower elevations and a device captioned to throw fire and burn the beams fill the right margin.
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Corner entrances with parallel and triangular outworks
At each corner of the circuit an entrance is screened opposite by two parallel walls, 30 to 40 feet long and 20 feet apart, running to the ditch embankments and armed with offensive positions throughout. Beyond them two more walls close into a triangle joining the outer line, ending in a bastion with gates, ravelins and ditches.
Solid corners and flanking towers for a square fortress
If a square fortress can be battered from two corners, those corners are made solid with sheds above, and the stronger corners carry towers with lateral offenses that defend the walls holding the solid angles. The entrance passes through one such tower, furnished with a ravelin and casemates for the ditches.
Mortar-proof tower with an oblique roof
Against mortars the tower's scarp rises in straight lines that converge to a point at its summit; from mid-height corbels run in a straight line to a corridor kept three braccia clear, with a parapet of three-and-a-half braccia and beveled crossbow-loops. The corridor is divided into as many rooms as there are loops, and the whole roof is left oblique so artillery shots cannot catch on it, the walls armed inside with thick bound oak.
Device to throw fire and burn timber
A mechanism sketched at the lower right is captioned as a means to throw fire and burn the beams, an incendiary device aimed at the enemy's timberwork.