Some extra influences and visual aids, thanks to Emma. All of the images are from Harry Potter, starting with the scenes from the Great Hall to the Whomping Willow tree to Green Gots Bank to the Ministry of Magic.
Figure 1 shows just how grand the size of the hall is, with the dark blue clouds adding to the effect.
Figure 2 has a good view of the hall in general, with a good sense of scale.
Figure 3 is influential to the design of the near-floating, encumbered mass of branches and vines, that is described in Room of Roots.
Figure 4 displays good lighting, as well as reflecting light, as well as perspective.
Figure 5 has good lighting and good sense of scale.