Well the "defensive wall" was locked in a technological struggle with gun powder since the invention of the early cannon. But for several hundred years forts changed to combat these inventions. (Sloped sides to deflect cannon balls, sand between layers of concrete to soften blows, split sections for fire control, etc.) and this went on and on around the world, from Ethiopia to Japan forts adapted to the weapons of the regions and time. But during the rapid industrialization period of the 1880s nations started to expand on multiple artillery concepts, like long range howitzers , large explosive shells, massive breach loading pieces that could fire rapidly etc. and while most modern forts could handle a few hits with these new weapons, they would crumble under excessive fire. But they were still used until ww1, by multiple nations and colonial empires. However WW1 made it very clear to all nations that forts of any kind have become obsolete in nature. Massive artillery barrages and such rendered them useless. And it would only get worse for the forts, with advancements in aviation and long range ordinances. A series of deep trenches and bunkers could protect against these threats more, along with allowing for quick counter attacks. That's why traditional forts were abandoned in Europe by WW2.