I think it'd be interesting, perhaps in war of kings to introduce the ability to walk on walls with units. Walls might take up 3 spaces, 2 facades and one walkway. Or you could condense into one space and just make it so melee can't attack units on walls till they scale it, takes a turn and reduces their attack and defense a bit. Units on the wall might lack the ability to attack units below them if they're melee but could have the ability added when they're to hurl a rock. This could have a set amount of damage that is effected by the unit's strength (attack) and can only be performed on enemy units who have not scaled the wall yet and are adjacent. Being on the wall could possibly render a bonus to attack and defense which would negate the need for a castle bonus.