First State National Historical Park is a National Park Service unit that encompasses several of Delaware's most significant colonial sites, including much of New Castle's historic district. The park was established in 2013 and covers scattered sites across Delaware and into Pennsylvania, but in New Castle it includes the courthouse, the Green, and the surrounding historic district.
The National Park Service presence means there are rangers and interpretive programs available, particularly at the courthouse. Admission to the courthouse museum is free, as is access to the Green and Battery Park. The park's interpretive focus is on Delaware's role as the "First State" to ratify the U.S. Constitution in 1787 — though in New Castle's case, the most compelling history predates independence by a century.