This castle is what I use as my server Headquarters. It includes many rooms full of chests for players to purchase items, an enchanting room, a furnace room, and the "executive offices". The P on the tower stands for Pandora, the name of the server. It took 3 days to complete by hand, and many days in planning. The following will help you replicate this design and use it for whatever in your Minecraft world.
Step 1: The Base, Entryway, and Interior.
I created a sidewalk leading up to the building, and I put two flags up (representing the server) and a P representing Pandora. The Base is a big rectangle, with 4 circular pillars in the corners. The pillars are a 11 block circumference circle. I first constructed the pillars, then connected them, making the base. It's 17 blocks tall, with a design of alternating blocks on the top. On the interior, I have carpet leading to the two towers, followed by a HQ sign and a help desk. The ceiling is made out of Glowstone.
Step 2: The Towers
The towers are 17 blocks in diameter, in the middle of the top of the base. They end up being 5 blocks away from the sides. Each tall tower contains 10 floors (3 blocks high), and is 60 blocks high in all. On floor one, there is a tunnel leading to the small tower in the middle. It is 13 blocks in diameter, and 6 blocks from the front of the base. That is where all the precious material is stored (diamond, emerald, gold, iron etc.) On the 6th floor, there is a tunnel leading to the tall tower in the middle. That contains the the furnace room and a huge enchanting room.
Step 3: The Top
On the 10th floor of the two towers, there is a bridge that leads to the executive offices. There are four offices, and you can build it to your specifications. This one is built for 4 offices, one for the Server Owner (me), Head Gamemaker, Redstone Master, and the Head Admin.
Step 4: Conclusion and Tips
All in all, you can take this design and tweak it to your liking. This building is really the masterpiece for my server, and I hope it can be for you too.
Here are some helpful tips to make this build faster for you:
1. Use WorldEdit. It will make building this faster.
2. Have other users help you. It will not only make it go faster, it's a good teamwork build.
3. Make it whatever you want it to be, and get creative with it! After all, it is Minecraft!