Instructions
Save several hundred thousand simoleons to pay for building supplies. The bigger you want your castle to be, the more simoleons you are required to save. A small castle should cost around 150 thousand, but a large castle may cost upwards of 500 thousand.
Enter build mode and flatten your entire lot by selecting the level terrain button. Click and hold your mouse on a corner of your sim's sidewalk, then slowly drag backward until the entire lot is highlighted. Release to flatten.
Click on the wall icon in build mode and create the base of your castle by clicking and dragging. For a basic castle, create a simple square by holding down shift while building your walls. For a more intricate base design, simply click and drag the walls where you want them.
Create additional levels by clicking the "level up" button on your toolbar and continue to build exterior walls until your castle is as tall as you want it. Go through each level of your castle and create your interior walls by clicking and dragging. Make sure to leave a space on each level for a staircase and landing.
Select your favorite type of stairwell by clicking on the stairs icon in the build menu. After selecting, click on the desired location to add the stairs, making sure there is room for a landing at the top. For a more modern castle, try adding an elevator, also located under the stairs icon. Repeat on each level of your castle except the top floor.
Click on the floor tool in the build menu and select your floor tiles. Stone-like patterns and tiles work well in a castle. Hold down the shift button and click to fill an entire room with the selected floors. Repeat with the walls, selecting paint colors from the wallpaper section of the build menu.
Locate attractive doors and windows under their respective sections in the build menu. Place them on your walls by hovering your mouse over the walls and clicking when they are in the correct position. Make sure each room has at least one doorway to make entrance possible for your Sims.
Select the water tool in the build section and choose the pond tool. Click and drag your cursor around the perimeter of your house in a circular shape to create a moat. Make sure to leave an area in front of the house clear of water so that your Sims can enter their castle.