At the time I'm writing this there are three answers that basically say "not much" and one "three advantages not to be dismissed". So I will try to give you a scalable approach. Most likely your players will tend toward the "not much" attitude and try to employ countermeasures or OOC-reasoning along the lines of those answers.
The point is: What do you want to achieve?
As others have noticed, it's hard to hide an army. But how does this apply to an army of darkness? If you want the defenders to be prepared, have them notice the army at any convenient time before they reach the fortications. However if the defender shall have little warning there are some points to consider. What if there are hiding places along the way? If your army doesn't need to breath even a lake would hide a complete army and get away from anyone trying to spy on them. Thick forests work out, if spying defenders are caught. Does the army of darkness tire? Maybe they can march off-road where no one would suspect an army to make such progress. Marching all day and night doubles the speed and should surprise any defender not used to fighting of such an opponent. (Reality's history: The french dismissed recon info as "the germans could never have made such progress in the short amount of time" at the beginning of WWII)
Sieges are boring. I mean deadly boring. That's the point. People outside the wall wait for people inside the wall to die (of hunger, not boredom but that's not any more interesting). But how does this change, if the people inside the wall know straight from the beginning that their foe can outlast them, because they are undead/demons that do not have to eat? The besieged will have to put up a fight early on. So you can have a short siege phase if you want to and then start a prepared counter attack.
Any counterattack of the defenders will happen at day. This is an unusual thing, because the attackers are probably outnumbering the defenders (why else would they start an attack?). Usually one might expect some stealthy counterattacks by defenders at night. In your case this wouldn't work out good.
Biggest Advantage (not mentioned by now)
The answers so far have addressed tactical/analytical advantages and some of them read like "How my character would counter it". But lets face it: People at war are not always analytical. One might think that imminent risk of death is a good motivation to fight "all out". In fact, people panic at certain points. The famous book "Art of War" by Sun Tzu is only some few pages long but dedicates a whole chapter on how to get your army into a position so that your soldier actually will fight (instead of fleeing or just defending themselves). Sun Tzu advises to get your own army cornered (in a tactical advantegous position)!
That long preface is meant to stress this point: Fear. The best weapon an army of darkness has is the deeply set fear of darkness in most people. RPG heroes are certainly not your average soldier, which can be good enough to reason why they often act fearless. Well would they be heroes otherwise? But an army gathers together whoever is able to hold a sword, spear or even fork. What will the average villager do, if he is forced to fight against an enemy that's lurking in pitch black night? Most likely he will huddle around fires or torches. Every torch bearer is one fighter less (average townspeople can't fight two handed with a bruning torch). And huddled groups are fair game for ranged attacks, which don't suffer from darkness as their target is lit up.
Plus, a single soldier does not have to look far in melee combat. What is it to you (undead or not) if someone 40 ft away needs your help , if there are 20 enemies between you? 60ft dark vision is a great advantage in melee. Since you defined there's fortifications the defenders can choose when to leave it and they will most likely make a counterattack at day.