Favorites, into battle! - 11.

no downsides.
the main thing is that both players can use this library upgrade (in case the second player finds the Tower).

That's exactly how it is - the library upgrade either exists (the checkbox is enabled) or the library is not upgraded; this setting is global, for the entire game and for all castles on the map. In my opinion, it's done perfectly.
So, a rather old rule, which was usually agreed upon verbally with opponents before the start of the game, could be written here as well.