NOTE: On the schematic it says that you need 116 black banners, but that is a glitch, so what you need is CYAN Banners.

The snowy fortress is a picturesque structure that stands out against the snowy landscape. Its design includes towering walls and spires, constructed mostly from mangrove, which give it a magical, wintry feel. Inside, you'll find cozy chambers, chandeliers, and hidden passages leading to other rooms, towers and balconies. Thrones and halls can also be found.