Right at the place where the Moroccan mountains come to a final end, and before Ceuta's Monte Hacho starts its ascending, the old remains of the old city walls are situated. Now it's in two parts, having a small canal going right through it, which creates a strong current.
An occasional boat passes. Some have a practical reason- getting quickly from the south side of Ceuta to the northern harbour. Others do it just for the fun of it, sailing through the canal with the 10 metre walls in good condition looks nice enough. At least when you see it from above.
The construction itself is believed to be from the Almohad period, but most of it was reconstructed in the 17th century.