Pro tip they forgot to mention: if you expect to need a medieval castle at some point in the foreseeable future, build it now, while concrete's available and cheap (and while construction efforts don't involve impressing a quarter of the skilled laborers in the country).
And don't use reinforced concrete; steel, even stainless steel, rusts over time, and the goal is to have something still serviceable a millennium later with ordinary maintenance.
And don't use reinforced concrete; steel, even stainless steel, rusts over time, and the goal is to have something still serviceable a millennium later with ordinary maintenance.