The Darb el-Arbain trade route, passing through Kharga in the south and Asyut in the north, was used from as early as the Old Kingdom for the transport and trade of gold, ivory, spices, wheat, animals and plants. Later, Ancient Romans would protect the route by lining it with varied forts and small outposts, some guarding large settlements complete with cultivation. Described by Herodotus as a road "traversed ... in forty days", it became by his time an important land route facilitating trade between Nubia and Egypt, and subsequently became known as the Forty Days Road. From Kobbei, north of al-Fashir, the route passed through the desert to Bir Natrum, another oasis and salt mine, to Wadi Howar before proceeding to Egypt. The Darb el-Arbain trade route was the easternmost of the central routes.