In the Middle Ages, Riga was a powerful port city, the largest city of the Swedish Empire in the 17th century, the leading port city of the Russian Empire in the 18th and 19th centuries, and the trading centre of the entire region. This status was strengthened by Riga's membership of the medieval Hanseatic League. Reminders of the Hanseatic period are the Great and Small Guilds, as well as the rebuilt Blackhead House - the sites for the craftsmen's communities in the flourishing city.