Hi,
My idea is do a Guild Castle skin thats look like the Atlas Castle
Maybe you can do this skin Expencive but bring in a npc who will give you a little extra buff like haidon (maybe 2% for all atributes) for Guilds that get the new skin.
When there is a way you can insert a way to craft with coin/cores items That you need to upgrade the Castle to this skin
best regards Narasch