Which Units Do Better On Walls?

[Evil_Maniac From Mars]

I'm finding sieges (even city sieges) more difficult than in Rome, which is excellent. However, even when I outnumber the enemy on the wall, I almost always lose. I usually hire mercenaries to take the wall, but they continue to fail, even if they massively outnumber the enemy. I've been auto-resolving sieges - even if I lose, I take less casulaties (which I know is because the resolve doesn't take walls into account). I'm at a loss, so I can only say that it must be the units. Which units do better on walls?