At first, Yule was only going to play around with the lizardmen and the mermaids! She was going to make them start a fight with each other and that would have been her entertainment! What was better entertainment than watching those weaker than you fight pointlessly?
But after her plan was ruined by that mermaid and vampire, she followed them back here and she saw those castles that just looked perfect for an invasion! She knew that this is the demon lord's castle and she decided that she would just piss him off a little by destroying his castle. She might also kill a few hundred demons in the process, but she was sure that the destroyed castle would piss him off more than anything else. Since the demon lord was no longer in Aradtie, wouldn't this be the best time to attack him?
Creation is hard, cheer me up!