This 14th century structure is rather fascinating and takes some work to find and follow, but it's worth it if you're up to it. Be ready to walk and climb and make sure you have a good map that you can follow. Parts of the area through which the wall passes have evolved as if the wall was not there; meaning there are neighborhoods and schools and such that hug parts of the wall, while other sections are more park-like and accessible. The area is also very hilly. Add to that the narrow, unnerving, steep steps to get to the top of the clock tower (featuring the oldest working clock in the city; 16th century), and you'll have a good cardio workout and then some. (The upper steps of the clock tower are tricky and not for everyone; do not attempt if you have any trouble walking, climbing, or have poor depth perception.) You can access three towers and walk on the wall in this same area. All in all, it's a bit of an effort to visit the Nine Towers and you'll be glad when you've finished, but you'll be happy you did it. And it's free.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.