A Plain Fortress of Double Walls Filled with Earth
Box-like double walls rammed with earth, an inner keep, and 80-foot defensive ditches
This page designs a towerless fortress for flat ground. Its first circuit is built as double walls, 8 feet thick outside and 4 within with a 12-foot gap, tied by cross-walls and great oak timbers so the whole stands like lidless boxes packed with earth rammed by pestles, as houses are built near Pisa, or better with gravel and lime where the cost allows. Within lie the garrison quarters and an open 40-foot space around the central keep, ringed by inner ditches 80 feet wide and 40 deep, with corner casemates and a drawbridge, and Leonardo warns that the defences must not be so low that ditch-water floods the caves or bombarded rubble blocks the casemates. Pen sketches show a keep on rising ground and a concentric square fortress plan.
On this page
Box-like double walls filled with rammed earth
The outer circuit is made of double walls, 8 feet thick outside and 4 within, with a 12-foot interval and cross-walls chaining them, bound together with great oak timbers. The result stands like lidless boxes joined head to head, filled with earth rammed by pestles, or better with gravel and lime where the expense allows.
Rammed-earth fill as built near Pisa
The hollow double wall is packed with earth rammed down by pestles, the very way houses are built in the district of Pisa. Gravel and lime would make a better fill, but Leonardo notes the cost often does not permit it.
Inner keep, corner casemates, and defensive ditches
Inside the wall are the garrison quarters and a 40-foot open space toward the central keep, then inner ditches 80 feet wide and 40 deep holding the keep and castellan's dwelling, with two corner casemates raking the ditch and a drawbridge to the keep. The defences must not sit so low that swelling ditch-water fills the subterranean caves or that rubble cut down by the bombards blocks the casemates at the waterline.
Sketches of the keep and the square fortress plan
A pen sketch at the centre left shows a keep or tower on rising ground reached by stairs, and a plan at the lower left shows a concentric square fortress with corner towers, matching the box-wall scheme. Together they illustrate the plain-ground fortress described in the text.