The flight of Dark Phoenix gets them to the Castle within an hour, giving them time for Cain to start setting up all the magical Cannons he's acquired and assigning Puppets to the defense force. With the extra Cannons they've gained from the pirate ships, he's got just over a hundred now, enough to give constant covering fire at every point along the walls. You don't need to be particularly strong to operate the Cannons; you need bodies, an area where the puppets excel.
Cain releases all his Companions from Merger and has them and their clones Summon their Bonded Forces, while Cain calls out the rest of his summons, including six Oath Breakers, since Danni is taking one of his base Legendary summons. That pair of Legendary Dragons won't be here for another hour, as they're flying high above the cloud cover to avoid detection.