"The Hundred" hundred is an historical or geographical term rather than a functioning administrative unit. In Delaware the hundreds are now used primarily for property descriptions in real estate transactions or for historical reference, not for governing.
When New Castle Hundred was created (in 1682), what is now the City of New Castle was the only settlement of any consequence along the Delaware River.
Roughly, it is bounded on the north by the Christiana River, on the south by Red Lion Creek, on the west by Salem Church Road, and on the east by the Delaware River.