Complex sentence: Its trade and commerce prospered and in the 15th century it attained the summit of its prosperity, ruling over a district about 300 square miles in extent, becoming a Free Imperial City with extensive territorial authority, and having a population of about 60,000.
Split into simpler sentences: Its trade and commerce prospered, peaking in the 15th century, ruling over a district about 300 square miles. It became a Free Imperial City with extensive territorial authority, and having a population of about 60,000.