The 1906 Antiquities Act gives the President the power to declare national monuments as an executive act. A National Park designation, however, is more significant because it comes with more funding and more wildlife protections.
Coleen Gentles writes on the City Parks Blog about why Fort Monroe is worthy of the designation:
"The site has the momentous distinction of being the spot upon which, in 1619, the first Africans destined for the British continental North American colonies landed-the vanguard of an estimated 10–12 million Africans forcibly brought to the colonies and, later, the United States.
Fort Monroe was begun in 1819 and completed in 1834. With a seven-sided shape, walls of stone, ramparts over a mile in circumference, completely surrounded by a water-filled moat, and bristling with huge artillery guns, Fort Monroe was given the nickname "Gibraltar of the Chesapeake." It is the largest fort ever built in the United States."