Castles create a different strategy. A walled city is an egg, you have a lot of area to attack and when you get through there's a treasure inside. A castle is a

Castles create a different strategy. A walled city is an egg, you have a lot of area to attack and when you get through there's a treasure inside. A castle is a pebble. It can be crushed, but you have to put in a lot of effort, you get nothing from it and there's a thousand more around you. IIRC the Song also didn't have great castle making technology. Didn't they do earthen fortifications reinforced with clay bricks? And weren't the walled cities typically sitting on plains, possibly without a moat? That's a lot easier to attack than a composite stone wall sitting on a mountain.