I initially found the description of this as a 'fortress' a bit misleading until I did a bit more research into what that means in Korea. Rather than a single castle, for example, or even a complex of buildings atop a hill, it really means a small city surrounded by a defensive wall. With that perspective, it was a bit easier to plan the best way to visit Hwaseong Fortress at Suwon, where what's left of the fortress is now mixed in amongst a modern urban sprawl.

For anyone planning this day trip from Seoul, I would suggest doing two things. The first is to walk the entire length of the walls that surround the fortress site - these are not just the main element of the World Heritage Site these days, but they are the most scenic, with fortifications, gates, and other decorative features. It took me about 90 minutes to do the loop (including photo stops). The other thing to do is visit the Hwaseong Haenggung Palace, a 'restoration' (reconstruction) of the palace within the city. I didn't find it as interesting as the walls, but together they add to each other's stories.

For any visitor to Seoul, I would recommend Hwaseong Fortress as a day trip (or half-day if you're efficient) even if you're not a WHS fan. It's easy to do and shows a different part of the history of the Joseon Dynasty, beyond the obvious World Heritage Sites like Changdeokgung Palace and Jongmyo Shrine. While you're here, there are a couple of interesting museums (including one dedicated to the fortress), as well as things like markets and the fun food streets you find all across South Korea.