Guild Castles are private areas for the guilds that own them. They are essentially the instanced version of the map a castle is located in without any monsters or portals (besides the one on the castle itself) where you're also not allowed to fly. The function of a guild castle is to simply act as a resting area so players don't have to go back to a city all the time. This means players can carry out some basic functions such as using Mail, repair their equipment and sell unwanted items to a Recycling Brother. Additionally, Guilded Blades can be obtained from a chest that spawns in each guild castle every Sunday at 8PM EST.