A drawing of four mortars firing into a fortress with crenellated walls; a section of the wall has been demolished and has fallen into the inner moat. Cannon have been ranged on earthworks either side of the breach, facing into the fortress to attack the defensive emplacements. Compare 912337 verso, showing a very similar arrangement, from the time of Leonardo's early work on the Battle of Anghiari, and thus c.1503-4; another in the Codex Atlanticus, 24r-a.

Melzi's number 157 at top left. Verso blank.