The Aghlabid Basins are a set of enormous 9th-century water reservoirs built by the Aghlabid dynasty to supply Kairouan with drinking water. The main basin is a circle nearly 130 metres in diameter and several metres deep — a structure that would be impressive in any era, let alone the 9th century. The complex functioned as a settling and purification system fed by an aqueduct running from the hills to the west.
The basins are on the northern edge of the medina, easy to walk to from the Great Mosque. They are open to visitors, and you can walk around the rim of the main basin. There is not much to do beyond looking, but the sheer scale of the ancient hydraulic engineering is genuinely impressive, and the setting — open ground, the medina wall visible behind, the basin stretching out — has a quiet grandeur. An information board explains the engineering logic. It takes about twenty minutes to visit.