The Ksiba ("small fort" in Arabic) is the more modest fort guarding the southern side of the old port, opposite the much larger Kasbah. Originally a fifteenth-century watchtower extended by the Ottomans and again by the Husseinids, it is the smaller of the two harbour forts and considerably easier to walk around in half an hour.
The interior is now occupied by the Oceanographic Museum of Bizerte, a small civic museum that houses tanks of Mediterranean fish, marine specimens, and a slightly worn display on the Bizerte Lake ecosystem. The aquarium is modest but the kids tend to like it, and the museum doubles as a sensible reason to climb up to the fort's upper terrace.
The view from the terrace looks straight across the canal to the Kasbah, and is the better of the two viewpoints over the old port. Open daily except Mondays; small admission fee.