From an April 27, 2006 Chosun Ilbo article:

When walking from Jongno1-ga to Dongdaemun, look left when you see the Dangdaemun Gate for a blue sign saying “Naksan Park 1.2 km.” Walk up along the fortress on your left to the top of the mountain. There is also a parking lot for those who are driving. If you think it is a little far to walk up to the top, you can take the No. 3 bus at a bus station near the Dongmyo Station of subway Line 6 or the Dongdaemun Station of subway Line 4.