Complex sentence: Imperial amphitheatres were built throughout the Roman empire; the largest could accommodate 40,000--60,000 spectators, and the most elaborate featured multi-storeyed, arcaded façades and were elaborately decorated with marble, stucco and statuary.
Split into simpler sentences: Imperial amphitheatres were built throughout the Roman empire; the largest could accommodate 40,000--60,000 spectators. The most elaborate featured multi-storeyed, arcaded façades and were elaborately decorated with marble, stucco and statuary.