To build a guild castle and all the items inside and out, it takes gold, rubies, guild runes, ore, wood, herbs, guild stones, etc.
It is quite and achievement to level the castle and even more of an achievement to place and level all the things you can have in and outside of your castle.
A castle can have things like a library where you can work off death debt, harvesting fields where you can gather wood, herbs, and ore, buff towers where you can get buffs and much much more.
Also there are things that can be placed in a guild castle that can help you in siege like processing factories where you can purchase flame towers etc during guild siege.
To me a Guild's castle is a measure of it's members and their dedication to their guild.