[Darvin]

The advantage of catapults and the like is that you can barrage an enemy from outside their retaliatory range (plus they're useful in a non-siege battle). The downside is that they take longer to use than siege ladders or towers. I don't know if defences will work in this manner, but if you can capture enemy walls, using ladders and towers would certainly be effective in preserving them for your own use (I'd love to see a campaign mission - preferably a later one since this is well suited as a high difficulty mission - to capture an enemy stronghold before you get pincered by enemy armies, then use its defences to defeat them).