I agree that orc walls seem lack luster. Though I have come to appreciate their walls after some sieges that showed they actually had a few upsides. For example paliside walls can not be climbed only knocked down. When your orc town has 2 layers of paliside walls the sum of the two walls is actually more health than a singular bigger wall. Another key mechanic that each orc fortress possess is the "Flood gating" of its gates that allow the orc to seperate enemy forces within gates and slaughter them one at a time. Combine all of this with the fact that orcs can spawn many many squads during a defense as well as the fact we have a much higher resource cap than the other two races, and we are actually fairly well off where the defense is concerned. Not to mention that as far as I have seen orcs have the most mounted siege weaponry that allows them to constantly hammer enemies with flaming projectiles no matter where they run unless they take out all of the outposts.
"To underestimate a Goblin Horde is to stare into the face of Hell and think it to be heaven." ~ Svulnar, The Goblin King