Step 1: The Base
First find a big area to build. Then build the front and backside of the castle 21 blocks long. And then the two sides are 16 blocks.
Step 2: Fill in the Frame
Step 3: Making the Towers
Make the walls 3x3 and make the tower 8 blocks tall and make sure there is an entrance if you want. Repeat this for each corner
Step 4: Now Make the Top
To make the tower look better make and extension one block out (5x5) and do the ledges like how I showed.
Step 6: Build the Walls
Make the walls five high. Probably the easiest step.
Step 7: Now the Roof
You can do the roof how ever you like. I put glow stone so that you can see inside the castle.
Step 8: Decorate
Now make the inside look amazing!
Step 9: Finished
Now you have a castle! Please like and subscribe and I will continue to make minecraft instrucatbles.