In my opinion walls are a bit too expensive, especially on carthaginians. It costs around 2000 stone to build a decent wall for an average civ, and around 4000 stone for carthaginians (I get it, they are three times as strong). But turtling isn't really a late game strategy, but an early game strategy. By the time you have put up those walls, the enemy has already rushed/boomed you long time ago. Because of the cost, people will rarely ever build walls because those resources can be put to much better use and battering rams quickly destroy your walls anyways. I think walls need to be cheaper like in AoE.