Decide on a LEGO castle design. You can sketch the castle design and layout before you begin to build, or simply start construction with an idea in mind and tweak it as you go. To make an original LEGO castle, you can modify existing LEGO castle kits or create one completely from scratch. Look at pictures of castles for ideas.
Construct the Lego castle, using a large enough platform to give you room to make the castle and a scene around the base. Gather your Lego bricks and build your original castle. It's really an art, so there is no specific step-by-step when it comes to the particulars of building your Lego castle.
Build the interior floors and add accessories as you build the castle. It's usually easier to build an entire floor at once (interior and exterior) before moving to the next story; often, this is the only feasible method.
Add to the detail of your Lego castle interior as ideas come to you. Read up on castle life for inspiration.
Create action scenes in your Lego castle; for example, Lego soldiers pouring boiling oil on intruding armies.
Make a landscape scene and include animals, such as a cow or horse, outside the Lego castle on the platform piece. Use trees and shrubbery pieces to make the exterior as realistic as the inside of your Lego castle.
See resources, below, for LEGO designs, books, sites, castle sets and more.