I flipped the picture horizontally to find mistakes. I recognized that the left side was lighter (Fig.15). The composition was not balanced. I had to widen the hole at the top of the cave; which helped to correct this (Fig.16).
The building on the right side was not good enough for me so I started to redesign it. I found a perfect picture on the internet (Fig.17). I placed it and I made some modifications (Fig.18). Then I returned to the arch in the middle of the picture to finish it (Fig.19).