But the biggest problem with Protective is with the AI Pillaging your improvements so even if you improved Wall and and castles that way... it'd still be considered underpowered in the human hands because of the pillaging.
I consider this bizarre. Every civ in the game is capable of preventing pillaging. Protective civs are not handicapped in any way.
The biggest problem with protective is that it provides a tiny fraction of its total bonus in the gunpowder era and beyond while every other military boosting trait spreads its bonus fairly evenly over all eras. Denying gunpowder units access to further drill promotions is the cause (though I'm not saying the fix is undoing this).