The Casbah of Algiers is a UNESCO World Heritage Site and the soul of the city. About 100,000 people still live in this dense labyrinth of narrow alleys, whitewashed houses and Ottoman-era palaces stacked up the hillside above the port. The Algerian war of independence started here, famously depicted in Gillo Pontecorvo's 1966 film The Battle of Algiers.
The thing to do is simply wander. You will find traditional artisans at work, small mosques tucked into corners, and views of the harbour through gaps in the buildings. Be aware that the upper streets can feel sketchy — it is best to visit during the morning, leave by mid-afternoon, and consider hiring a local guide.