I love trying to design my own lego sets and finally created a castle for my kids to play with. I buy the pieces individually from bricklink (a great online lego marketplace) and typically either sketch out a design first or just start building. For this particular piece I didn't really have anything in mind when I starting building, I was basically just going for "castle." The biggest challenge was making it modular. I really wanted something that could come apart/be played with.