The smaller of Kairouan's two Aghlabid pools has none of the drama of its bigger neighbour, and that is part of why you should stop. It is a polygonal settling tank, about thirty-seven metres across, built in the ninth century to catch the first rush of rainwater and let the mud sink before the clean water moved on to the main reservoir. Engineering of this kind is usually invisible, and here you can see how it worked: the channel entering at one side, the stepped sides, the tower in the middle that once held an observation platform.
The surroundings are modest. A fence runs along the edge, a few trees give a bit of shade, and the pool sits among streets of low concrete houses, with kids kicking a ball on a patch of dust nearby. After rain, the water turns the colour of tea. During long dry spells it shrinks to a puddle, and you can walk around the dry bottom to see the stone floor. Most visitors spend ten minutes here, and ride on to the larger basin or into the medina.
The roads around the basins are quiet, with narrow tarmac, patchy edges and the occasional donkey cart. In a city of this size, traffic is mostly shared taxis and small vans, which stop without warning, so give them space. Come in the morning, before the sun gets harsh, and plan to leave by midday in summer, as temperatures can reach forty degrees. Wear a hat, carry water, and keep your phone camera ready for the late light, which is soft and golden.
Kairouan itself is a good base. Fuel stations line the main approaches, food is cheap, and many guesthouses in the medina occupy old merchant houses with courtyards. The famous Great Mosque is a short ride away, along with the Barber's Mosque and the carpet workshops. From here the road east to El Jem is flat and fast, with olive trees stretching to the horizon, and the amphitheatre waiting at the end like a surprise.