Partly because I assumed that that's the only way to get to the very top of the tower. But mainly because I wanted to draw King Sombra and stairs. Really there wasn't much thought put into this. I was just bored and felt like drawing villains.
Sombra: For too long my people have depended on this... bauble for their strength. They need to seek the strength within themselves. All this has done is make them dependent on it for comfort. I know the Princesses will disagree with this method, but they haven't seen the dependency my subjects have to this.