Figured out a decent way for human players on higher difficulties to actually be scared of an AI attack after you build walls. I inserted the following lines in the UnitAbilities xml file (line 130)
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_MELEE"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_ANTI_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_WARRIOR_MONK"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_HEAVY_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_LIGHT_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_NAVAL_MELEE"/>
Now units of all types act like siege towers. On a few autoplays, the AI's started steamrolling and walls acted more like those from Civ 5. You can adjust the wall defense hit points as you deem necessary. Cavalry still take a beating when attacking but at least they do good damage.
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_MELEE"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_ANTI_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_WARRIOR_MONK"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_HEAVY_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_LIGHT_CAVALRY"/>
<Row Type="ABILITY_BYPASS_WALLS" Tag="CLASS_NAVAL_MELEE"/>
Now units of all types act like siege towers. On a few autoplays, the AI's started steamrolling and walls acted more like those from Civ 5. You can adjust the wall defense hit points as you deem necessary. Cavalry still take a beating when attacking but at least they do good damage.