Fort Casimir Fort Casimir was a Dutch settlement in 17th century colonial province of New Netherland . It was located on a no - longer existing barrier island at the end of Chestnut Street in what is now New Castle , Delaware . The trading post was named for Ernst Casimir of Nassau - Dietz , count of Nassau - Dietz and Stadtholder of Friesland , Groningen and Drenthe in the Netherlands . Background . The Dutch called the Delaware River the " Zuyd Rivier " , or South River , and considered the lands along it banks and those of its $ 3 to be the southern flank of its province of New Netherland . In 1638 , the Delaware Valley began to be settled by Swedes , Finns , Dutch , and Walloons and became the colony of New Sweden , though this was not officially recognized by the Dutch Empire . Sovereignty . The fort changed hands several times . Shortly after his arrival to take up his position as up as Director - General of New Netherland in 1648 , Peter Stuyvesant , attempted to re - assert control of the region and its lucrative trade . In his initial attempt at control , he effected the construction of Fort Beversreede near the terminus of the Great Minquas Path , at the mouth of the Schuylkill . The location of earlier - built Fort Nassau on the east bank ( now New Jersey ) of the river , had proved disadvantageous since the richest fur - trapping area of the native Susquehannock and Lenape populations was inland to the west . In 1651 , Stuyvesant had the fort dismantled and relocated to the western bank downstream from Fort Christina , the first and one of the larger Swedish settlements . He named it Fort Casimir . Fort Beversreede was abandoned and the Dutch presence was consolidated . On Trinity Sunday in 1654 , Johan Risingh , Commissary and Councilor to New Sweden Governor Lt . Col . Johan Printz , officially assumed his duties and his attempts to expel the Dutch from the Delaware Valley . Fort Casimir surrendered to the Swedes and was renamed Fort Trinity ( in Swedish " Fort Trefaldighet " ) . On June 21 , 1654 , the native peoples met with the Swedes to reaffirm their alliance . Stuyvesant led a Dutch force which retook the fort on September 11 , 1655 , renaming it New Amstel ( in Dutch " Nieuw Amstel " ) . Fort Christina , located to the north fell on September 15 , 1655 . Fort Nya Elfsborg , on the east bank , was abandoned and set afire by departing Swedish forces . New Sweden came under the control of the Dutch . John Paul Jacquet was immediately appointed Governor , making New Amstel a regional capitol of the Dutch province , In 1664 , Stuyvesant peacefully surrendered control of Fort Amsterdam , and thereby , all of New Netherland to the British . Proceeding south the British peacefully took Fort Altena . A symbolic resistance was offered at Fort Casimir , but the fort quickly succumbed . In 1673 - 1674 it came under Dutch control again , but reverted to the English after the signing of the Treaty of Westminster . The fort was abandoned around 1675 and the site was used for several different commercial purposes . Low laying areas around the old fort site were filled with ash . It is believed that presently a parking lot and several houses are located above the remains of the fort . A preliminary archeological survey of the area was conducted in 1986 .