At the greatest extent of the Empire, the southern border lay along the deserts of Arabia in the Middle East and the Sahara in North Africa, which represented a natural barrier against expansion. The Empire controlled the Mediterranean shores and the mountain ranges further inland. The Romans attempted twice to occupy the Siwa Oasis and finally used Siwa as a place of banishment. However Romans controlled the Nile many kilometres into Africa up to Syena, Berenice, Hyerasykaminos and even Qasr Ibrim, (the southernmost of all), near the modern border between Egypt and Sudan, then Meroe, lying very near the tropic. The period in which each aforementioned town represented the final frontier of Rome is uncertain.