The first sketch played with the idea of Howl's head emerging from the castle as it fell apart. Howl, even in creature form, is an attractive character so this look didn't appeal to me as the form looks strange, bulbous. It's also difficult to tell that it's Howl.
Smoke is always emanating from the castle, so I played with the concept of Howl emerging from the castle as a cloud of smoke and then juxtaposing Sophie in the ground with her face forming from rock formations. The two elements also reflect qualities of the characters. Howl is always about freedom and fancy, while Sophie is grounded and humble. Her disposition is more resolute and Howl's is more whimsical.
I wanted the composition to feel more interesting. This was just exploring that idea to see if it made the design stronger. However, I was still struggling with Sophie's face getting lost in the ground so I dropped the concept.
I selected this as the final design, retaining elements that I liked the most from the previous versions. I added more smoke and destruction to the castle along with Sophie and Calcifer waiting calmly on the hillside. I wanted the rock formations to point subtly towards them and draw the viewers eyes since they may blend into the background.