Der Kestle claims that this is its first Solstace.

At first, I thought, yes - because it was rebuilt by Agatha after being completely shut down. But she restarted it with a part of the castle that had been transferred into Otilia's body. Surely it had seen other Solstaces.

Unless, of course, it is just talking about the joining in. In which case the question is who the "they" were, and how (and indeed, why) the castle was barred from the show before. Probably the Heterodynes and a direct order.