I'm having the same problem.

Still getting used to M2's mechanics, I tried several custom castle defenses the other day.

Each time, I gave the enemy 2 towers and a ram, but I cannot remember my archers (plain longbows but with maxed out experience) destroying the enemy's siege equipment! All there was (out of maybe 5 battles) was a single siege tower!
Mind you, I've been defending a huge city, and maybe its walls are higher (I mean, an archer would always have to shoot upwards).

Funny thing is, I've had some castle battles in my grand campaign, but with less developed towns, my archers never had a problem firing teir arrows in a flatter trajectory.