My suggestion - Be sure to use decent melee troops when using either towers or ladders. Swords and axes are more preferable to spears and halberds. In my current game, I've been using Hashishim to storm the walls and they're doing very well. Expensive but very effective. And they seem to shrug off arrows most of the time.

I agree that towers reduce casualties more than ladders. But! Towers are soooo slow, they're most often burnt down before they even reach the walls... And it takes time for the men to assemble and deploy.

On the other hand, when assaulting a huge city with a central keep, ladders are more useful as you can re-use them - carry them through the outer gates and scale the inner walls. I usually use archers for this since the enemy AI hardly ever mans the inner walls.

Instead their General and remaining units stand in the middle of the square and gets peppered by my archers on the walls.

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I have a query though:

When placing archers on walls as the defender, can we place more than 2 units on the same stretch of wall?

And: Will they be more effective? Or will they end up shooting each other (via friendly fire)?