The Rhumel Gorge is the reason Constantine is Constantine. A river has cut through the limestone plateau over millions of years, leaving the city perched on a great rock island with near-vertical walls dropping 100 metres or more to the river below. Seven bridges and viaducts span the gorge at various points; the cliff faces are studded with cave openings that were used as dwellings into the 20th century. The gorge is polluted — rubbish and wastewater accumulate in the narrow canyon — but the scale and drama of the setting override the grim details.
The best views of the gorge are from the four main bridges: the Sidi M'Cid suspension bridge (highest), El Kantara (most central), the Mellah Slimane bridge (midway), and the Sidi Rached viaduct (most imposing at close range). Walking between bridges along the plateau edge takes about an hour and gives a complete sense of the city's extraordinary geography.