The message that reached Dulla and her undead army through a bat familiar was that the demon king had accepted the terms for their meeting! They would be having a [Truce Meeting] soon and Floid decided that it would be best to have the meeting in Aquinas. Since that was now Vampire territory, the undead would be more willing to go there. Floid knew that the undead would never come near his castle because every demon was afraid of it. The demon lord's castle was a symbol of power and anyone that came near to it would immediately feel scared and guarded and they would never feel like going inside! The feeling wasn't because of any curse, but the castle just had an aura of oppression around it that would make any of the demon lord's enemies surrender! That was why he chose Aquinas instead.