Fort Casimir was a Dutch settlement in New Netherland, located in what is now New Castle County, Delaware. Built in 1651, it was taken by Johan Rising of New Sweden three years later, on Trinity Sunday, renaming it Fort Trefaldigheets ("Fort Trinity").
On September 11, 1655, Peter Stuyvesant, leading a Dutch force, retook the fort. [1], calling it New Amstel.
Possibly named for Ernst Casimir of Nassau-Dietz or his successer, Henry Casimir I of Nassau-Dietz, both 17th century counts of Nassau-Dietz and Stadtholders of Friesland, Groningen and Drenthe.