Well, problem with purity here is that you might need many different materials to reinforce stuff, and only iron and stone has purity now. This would make some buildings naturally easier / better to reinforce than others. Of course you can make iron the common material for all reinforcements, but tbh... that's boring imo

As for the walls... I think that higher tier walls are the natural way to improve them. As you advance in the game and gain access to new materials you can build new and better walls. I proposed reinforcing as a way to upgrade your infrastructure, as it currently lacks such feature. So instead of adding new tiers (and graphics) of all those smelters, houses, kilns, ovens etc. and then having to destroy and rebuild them you would simply reinforce them.