A basic fort is usually made by constructing four walls around yourself, building a ramp up to the second floor, and then repeating this process. Widening your fort will keep yourself better hidden inside.
At the top of your base, place nine roof structures around the opening to better shield you from incoming snipers and grenades. This also allows more freedom to move around during combat situations.
Make sure to stay unpredictable. Make irregular walls and randomly placed ramps to keep intruders confused. Keeping your fort disorganized could make for some sweet trap locations for your enemies to wander into.
Edit a structure before you place it to make a preset structure. Use it to create arched walls for a support system or rows of windows for better vantage points.
Remember that you are safe up to three stories high. A fall from any higher will result in damage.