From the WHC Inscription:
It is an exceptionally well-preserved example of a small-scale trading port active the 15th to 19th centuries which traded widely, both with the countries of Southeast and East Asia and with the rest of the world. Its decline in the later 19th century ensured that it has retained its traditional urban tissue to a remarkable degree.
The town reflects a fusion of indigenous and foreign cultures (principally Chinese and Japanese with later European influences) that combined to produce this unique survival.
Read more: http://whc.unesco.org/en/list/948