If you wish to see any fortress near Seoul, I would recommend Hwaseong Fortress in Seoul.

It's listed as one of the UNESCO world heritage, and if you are UNESCO sucker like me, you would love to drop for a visit at this place.
Located outside Seoul, probably one and half hour driving from Seoul and takes half day to visit.

There are two options available :

1. Walking along the fortress 
Walking is a great and fun way to explore the fortress, as you can view it all while learning history.
Lots of stairs to climb. However it need a full day to explore.
Think 5.74km to do stair climbing...

Or

2. Take a tram ride (Lazy like me, however faster and efficient.)
Down side - specific timing, not much exploration can be done.




                      Bus 2, 7, 7-2, 8, or 13 and get off at Jongno 4-geori (intersection).
                      Walk 5 minutes to fortress.