Back to the foreshadowing I mentioned earlier
. Someone said that it was the Doom Bell. I was thinking about this, and I think otherwise. The Doom Bell woke up sleeping monsters and let the Jägers in the city, but Agatha remained relatively in the background. Now if the Castle (and Agatha) could pull off something that would drive off the Baron's forces, that would certainly destroy the Empire and make all Europe recognize Agatha as the new Heterodyne. It would also require so much power that would probably deplete all the Castle's reserves, effectively killing it, therefore fulfilling the foreshadowing.