Aki is a small city on the Pacific side of eastern Kōchi Prefecture, easy to miss if you are racing between Kōchi and Cape Muroto. It is not a big-ticket stop, but it has a compact cluster of old Tosa sights that make a good half-day if you are already in the region.
The best walk links the Samurai Quarters, the Nora-dokei, and Aki Castle Ruins. The streets are quiet, the scale is human, and the old walls, hedges, canals, and field clock give a sharper sense of rural Tosa than a rebuilt castle town would.
Free rental bicycles are sometimes available near Aki Station, which is useful because the sights sit just far enough apart to make cycling more pleasant than walking in summer. Treat Aki as a gentle local stop, not a destination to cross Japan for.