Complex sentence: The state gave Frost Point, on which Fort William and Mary stands, to the Federal government in 1791 to protect the Portsmouth and the 1800 Portsmouth Naval Shipyard, and the fort was rebuilt.
Split into simpler sentences: The state gave Frost Point, on which Fort William and Mary stands, to the Federal government in 1791. To protect not only the Portsmouth area but the 1800 Portsmouth Naval Shipyard, the fort was rebuilt.