The surviving city wall is a reminder that Nakhon Si Thammarat was once a defended regional capital, not just another long Thai provincial town. The old fortifications enclosed about a square kilometre, though today the main visible remains are the restored northern gate and a short stretch of wall.
It will not take long to see, but it helps orient the old city. Pair it with a walk down Ratchadamnoen Road towards Wat Phra Mahathat.