Complex sentence: He has suggested that these screens were one of the most important elements in the throne hall, and that this formalized landscape illustrates the Joseon political cosmology (the Theory of Yin and Yang and the Five Elements).
Split into simpler sentences: He has suggested that these screens were one of the most important elements in the throne hall. That this formalized landscape illustrates the Joseon political cosmology (the Theory of Yin and Yang and the Five Elements).