Intersection of Delaware and Washington street showing nemours building. Rodney Square showing Public Library and Delaware Trust Bldg. and Hotel Du Pont. Rose Garden, Brandywine Park. Delaware race track and grandstand. Wilmington is the largest and most important city in the state of Delaware. It was founded in 1638 by the Swedes. The city was laid out in 1731 and now has a great number of fine buildings, wide streets and parks.