Complex sentence: St. Mary's Guild, who built the Guildhall, became the richest and most prominent of Boston's Guilds and its members included many of the town's merchants and traders at a time when the town itself was enjoying great prosperity.
Split into simpler sentences: St. Mary's Guild, who built the Guildhall, became the richest and most prominent of Boston's Guilds. Its members included many of the town's merchants and traders at a time when the town itself was enjoying great prosperity.