In a seperate universe in which the Siege of Lordaeron was actually thought through and written properly, this would have meant that it got scrapped and didn’t happen at all.
Seriously, during the dark ages one of the greatest tactics waged against your enemies would be siege warfare. Any defensive structure or fortified city would be incredibly risky to make a full attack against, because a defender’s advantage could give any army the ability to defeat their enemies even when outnumbered 10-to-1. However, if your army was big enough to stand in front of all of the gates and stop your enemy from bringing in food or water, they will inevitably surrender. So, if the undead can’t be starved into submission, they can’t be sieged, and armies will have to risk their forces dying in droves for a chance of victory. That makes them powerful and scary.
Unless, of course, portals are actually viable enough in the WoW universe to move entire armies through and you can just place them behind the enemy walls, in which case why would any settlement bother to have big, expensive walls?