The wooden walls are actually very useful for resisting sieges.

Yes, they'll break like paper and it's hard to fire your missiles over them, but that's not the point; they create battlenecks the enemy will have to squeeze through to get at you. That means if you have a phalanx unit defending it, they're extremely hard to kill, because the enemy will be in disarray when moving through the gap, and they wont be able to flank you either.

I have defeated a Macedonian army about 4 times my size defending wooden walls with my Greek army, which was made up almost entirely of infantry(hoplites and some javelins), with only the general being mounted and no archers.