Complex sentence: Sunken relief was therefore used on harder, more difficult stone and when the builders wanted to finish quickly, although it was also appropriate for exterior surfaces, where the shadows it created made the figures stand out in bright sunlight.
Split into simpler sentences: Sunken relief was therefore used on harder, more difficult stone and when the builders wanted to finish quickly. It was also appropriate for exterior surfaces, where the shadows it created made the figures stand out in bright sunlight.