I think he meant in real life, not in AoE2. And in real life the advent of cannons changed how fortifications had to be built, moving from tall and thin castles to less tall and more thick walls.
Trebucket is also useless against stone fortifications. Kings and Generals is reliable only on general knowledges. Other than that, events they told are deficient, falsified or hoax. For instance, I looked Battle of Manzikert video and video omitted most important clauses of peace treaty. In treaty, Byzantine Empire become vassal state of Seljuks even though Byzantine Empire violate this treaty afterwards. Secondly, Romanos and Alparslan dialog is longer than Kings and Generals told and it indicate different meanings. In conclusion, Kings and Generals isn’t reliable source.