PARIS The discovery of a 4,000-year-old fortified town hidden in an oasis in modern-day Saudi Arabia reveals how life at the time was slowly changing from a nomadic to an urban existence, archaeologists said on Wednesday. The remains of the town, dubbed al-Natah, were long concealed by the walled oasis of Khaybar, a green and fertile speck surrounded by desert in the northwest of the Arabian Peninsula. Then an ancient 14.5-kilometer-long wall was discovered at the site, according to research