There's a reason he waited for nightfall before attacking, and it isn't better night vision.
Among the Grand Prince's necrofeudal vassals are certain incorporeal undead, spectres and shadows and wraiths searched out at great expense or created by dark magic to serve him. On their own, none of these undead are... plotters. Ambitious. Undeath so often dulls the mind, weakens the convictions; it takes a necromancer a hard hand and stern mind to bind them to service, to drive them out of their petty dens and into the hunger of the night.
But give them life to feed on - life that is huddling behind physical walls, life that was left without its clerics because they were more desperately needed in the front lines, life that is weak and physical and ill-trained, militiamen with none of the magic weapons the finest soldiers of a true army would bear to protect them from this -
- and they can replicate themselves, the greater making the lesser, each bound to their creator's service to the limits of its powers.
In the darkness, the ghosts of men are invisible, especially to an enemy with poor night vision. And so now the spectral tide rushes the barred gate, passes through and there is life, life unending with little wood bows and little wood crossbows, all crammed into so little, little space -