Really the only thing that made defensive walls useful for sieges in that era was the fact no one had any decent portable fireproof siege towers, so most towers ended up burnt down, unless the city under siege literally ran out of things to light on fire and toss into the (wooden) tower.
Also see "moat".
Defensive troops on the walls were pretty good at tipping towers over with ropes or whatever, too.
You'll note like a good engineer he promises all the easy stuff like logs being pretty good at stopping arrows, and doesn't mention the unfixable parts like "it'll burn" or "it'll tip over".
Really the only thing that made defensive walls useful for sieges in that era was the fact no one had any decent portable fireproof siege towers, so most towers ended up burnt down, unless the city under siege literally ran out of things to light on fire and toss into the (wooden) tower.
Also see "moat".
Defensive troops on the walls were pretty good at tipping towers over with ropes or whatever, too.
You'll note like a good engineer he promises all the easy stuff like logs being pretty good at stopping arrows, and doesn't mention the unfixable parts like "it'll burn" or "it'll tip over".