An undead assault would definitely make the threat feel more real. And the magical ballistae things Arhu made, I want to see them in action! There's a mod on the workshop that adds a sort of undead army, but it looked a little excessive.
I'm not sure if there has to be four generators that resurrect the undead, or any at all. One or two at most. A resurrecting machine so early on would tread dangerously close to creating a fail state if all the guards helping you died and there were an excess number of undead (unless you made them weak enough that you could handle them by yourself.)
I think like five or six undead attacking the north or west gate at once with two or three guards (and those ballistae) to help you out would be exciting enough and get the message across. You get the feeling a little bit from the orcs assaulting the beach as you arrive, but otherwise the orcs hardly feel like they are threatening the town either.
