The city of Tunis is built on a hill slope down to the lake of Tunis. These hills contain places such as Notre-Dame de Tunis, Ras Tabia, La Rabta, La Kasbah, Montfleury, and La Manoubia with altitudes just above . The city is located at the crossroads of a narrow strip of land between Lake Tunis and Séjoumi. The isthmus between them is what geologists call the "Tunis dome", which includes hills of limestone and sediments. It forms a natural bridge and since ancient times several major roads linking to Egypt and elsewhere in Tunisia have branched out from it. The roads also connect with Carthage, emphasizing its political and economic importance not only in Tunisia but more widely in North Africa and the Mediterranean Sea in ancient times.