"What are they trying to do? Gather more forces?" This was the only explanation Daniel could think of. Thalassar likely felt unprepared to attack Camelot directly and had instead decided to target the neighboring domains first. By doing so, he could gather more corpses to bolster his army, using them as cannon fodder for an eventual assault on Camelot.
The lord of the targeted domain quickly drafted their people to defend against the undead. At the same time, they sent a horse messenger to Camelot to request Arthur's help, offering to become his vassal in return.
However, it took two days for the horse messenger to reach Camelot Castle and another three days for Arthur to gather his knights and march to aid the besieged domain—time the people of that domain couldn't afford to wait.
Morgan le Fay also volunteered to accompany Arthur to help fight off the undead and protect the king.