Can Legendary Warrior and Behemoth members talk about the breaking of their alliance outside of this thread in your own guild forums? If you want to work out details/compensations or other kinds of discussions, surely the two guilds probably want to do it privately.
"This is a joint castle": No, it's Behemoths.
Can you go to guild dungeons as a LW member using your "alliance castles"? No, only Behemoth members can go to guild dungeons using the Prontera switch. You can be "allied" and form alliance/parties/best friends or whatever. You can even marry Amara in real life for all the game cares. But the switch will never let someone who is not a Behemoth member into the guild dungeon using Prontera Castle. Likewise, castle reset is also something completely unique to the decision of the Castle owning guild leader. Prontera, to the views of both the game and the GMs, belong to Behemoth.
Rather do they "deserve" or "do anything" or "invest enough" is another matter that's for discussion between LW and Behemoth, but none of that matters for who is the castle owner.
The way I see it is that *all* of this drama is related to the breaking of in-game player alliances and how things are not working out for both sides. I don't see another other members of the community, and even a few LW members who posted in this thread, disagrees with me that as of today, Behemoth is still the legitimate owner of Prontera castle and thus have the right to decide if they want to reset their castle and hand it over to the GMs or not.