Every castle upgrade currently unlocks a game mechanic. Level 2 unlocks alchemist Level 3 unlocks mage (and transmutation) What would level 4 do if it was unlocked? Would its requirements be?
I'm assuming this was what would its requirements be? Well put answer though as a castle upgrade for stats alone isn't really necessary with equipment, charms and allies all providing static stats already so a new castle is only really necessary with a new feature.
Support. Or have it automatically UG based on stats or rank or something like that. Similar to events where top 10 stringers get a certain type of castle, top 100, top 1000 etc Stat increase might be minimal but offer a point of difference to the gem in each persons kingdom
Maybe the dev's could release the new upgrade for the castle to introduce the gifting system. Just a thought but they need some way to release it.