It is one of many technical design pieces that he completed for various patrons during his lifetime alongside the artworks and ink drawings for which he is now best known.
Other technical drawings completed by Da Vinci include early plans for a device with some similarities to a modern day helicopter and an early and relatively primitive machine gun.
One thing almost all of these technical drawings have in common with one another is that they never led to an item being produced.
Da Vinci appears to have treated these sketches as more of an intellectual exercise than an attempt to actually manufacture a finished product.
Siege Defences is dated 1481 which makes it one of the earlier surviving Da Vinci sketches, many of the more complex sketches were not completed until after the turn of the Century. Unlike some of Da Vinci's better known sketches, such as that of the helicopter or arial screw, this is a relatively detailed and technical drawing.
There are copious notes along the top of the page which detail the working of the device. The drawing is also notably different from other drawings, such as that of the spring device in that it shows close up, technical details of the device and its function, rather than focusing almost entirely on its functionality.
As with so many other of Da Vinci's creations, screws feature heavily in the sketch. It has been suggested by some contemporary art critics that Da Vinci's fascination with screws came from his love of depicting nature in some of his earliest sketches and paintings. The shape of screw which he uses in this drawing is recognisable from one if his earliest landscape sketches.
Accounts as to why Da Vinci produced so many of these drawings have varied. It seems likely that the artist saw them as an opportunity to exercise his brain and turn over complex problems in his head. It's clear that Da Vinci had no part or particular interest in taking his ideas beyond the drawing board.
What's less clear is whether anyone else did. Whilst there is no trace of the inventions from his most famous sketches ever actually having come to fruition, it is also the case that some have been built retrospectively and found to function without issue. Whether the problem was lack of interest or the technical limitations of renaissance engineering is likely to remain a mystery.
Siege Defences is an interesting expansion of Da Vinci's collection of sketches that cover military hardware. It helps to fill in yet another detail of Da Vinci's career and the jottings towards the top of the sketch help give us more insight into the artist's thoughts on how the device would actually be put to use than is available from some of his other sketchings.