Fortress sections and a double spiral staircase
Cutaway views of a round keep and defensive walls, with a lumaca doppia
The sheet gathers two cutaway studies of a fortress: a large central section through a cylindrical keep, its crenellated wall and internal fittings, and a lower angular section showing a staircase running up a bastion behind a moat. At upper left a double helical staircase is drawn inside a rectangular shaft and labelled a 'lumaca doppia' (double spiral). Drawn faintly in metalpoint and ink, the studies belong to the military and architectural investigations of Notebook B. Further faint, untranscribed writing is visible in the lower-left margin.
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Angular section through a round fortress keep
The dominant drawing cuts away a cylindrical tower to expose its interior, ringed by a crenellated curtain wall and outworks. The caption names it simply an angular section of a fortress.
Double helical staircase (lumaca doppia)
At upper left, twin intertwined spiral flights climb inside a tall rectangular shaft roofed by an arch. The note beside it reads simply 'double spiral'.
Second fortress section with internal stair
A lower study shows another cutaway of defensive works: sloping walls and a moat with a straight staircase mounting a bastion. It is captioned as another section of a fortress.