Complex sentence: His rule spanned c.380--413/415 CE, during which the Gupta Empire achieved its zenith, art, architecture, and sculpture flourished, and the cultural development of ancient India reached its climax.
Split into simpler sentences: His rule spanned c.380--413/415 CE, during which the Gupta Empire achieved its zenith. Art, architecture, and sculpture flourished, and the cultural development of ancient India reached its climax.