The treasury inside your guild castle can be found directly ahead, inside the bottom floor of your castle. In fact, when you first obtain your castle, you can see the treasury through the missing inner gate!
Run on inside, you know you can't wait.  What you are going to be looking for to access your treasury is a small switch to the right of the large circular treasury chamber portal.
Interact with the switch and you will open your guild castle treasury. Here you can store 100 items on the first page that's available to you.
At the bottom left on the treasury window you will see three small square buttons. These are your treasury controls.
The first button is "Access Log". This will open a window displaying item deposit and withdrawal records. (Question for gamemasters, how many log entries are saved/displayed in this log?)
The middle button is "Permission Setting". This will open a window containing access information for each rank of your guild.

• "Browse Permission" is access to look into the treasury.
• "Placement Permission" is access to placing furniture within the guild castle. Note: A person may not pickup, move, or rotate furniture that they have not placed inside the castle themselves. This currently includes the Guildmaster! The Guildmaster does, however, have a special button visible on every furniture item in the furniture window to "Return this item to it's owner's mailbox".
• "Grabbing amount per days" is a text field containing the number of item slots in the treasury that a person of that rank may remove from the guild each day. (This number may be 0 to 99.) If an item slot holds 1 item (a recipe, for example) it counts as 1 "grab" for the day when removed. If the slot holds 100 ore, it still counts as 1 "grab" for the day when someone removes the stack of 100 ore. Note: Regardless of the number of "grabs" allowed per day, characters are allowed an apparently unlimited number of deposits per day. Please also note: It is not currently possible to split a stack that is in the treasury, and only take part of it. You will have to remove the entire stack, split it to take what you need, and return the remainder.

As mentioned previously, all 3 of these settings are set on a per rank basis, so don't miss out on anything when you're setting them. Note: All three of the above settings are set on a per page basis, also, so you must remember to set these again when you purchase additional storage pages for your treasury. (See below for purchasing additional pages.)

The final button is "Purchase new page". This button will open a new window displaying the resources required (in your guild resources page, as when obtaining the guild castle) in order to add new pages to your guild treasury. (At the time of this posting, the first additional page of storage was set to cost you 535,800 gold, 0 guild rubies, 1950 ore, 1950 wood, 1950 herbs, 4800 guild runes/magical essence, and 0 champion's core.)

Known Bugs: When you have the treasury open, closing the window by the X button on the top right does not reset the treasury "switch". (Remember the switch? To the right of the treasury door?) So you will need to interact with the treasury switch twice to open the treasury if the switch is in the "open" position.

A FINAL WORD OF CAUTION: It is incredibly common for guild members in online games to rob the guild blind if given unrestricted access to a guild's treasury. Exercise extreme caution and prejudice when assigning access to your treasury to your guild ranks. And never give unrestricted access to new recruit ranks. Historically, they are the most likely to rob your guild and disappear into the night. Though they may have wrongly taken any number of items, and your "access log" can prove who took what, and how long ago, the Guildmaster did give them permission with those settings to take things, so you can not expect the GMs to reimburse you for any lost items. Beware!