On my many trips to the airport, as I passed over one of the westernmost bridges on the Han River, I have often looked back north to notice a hill by the river with a pagoda on top. On my many trips to Ilsan I had also noticed signs for the Haengju Mountain Fortress. I only just realized recently that these two were the same place.
Just beyond the main gate is statue of General Kwon Yul, who led his force of 3,000 against some 30,000 Japanese troops. His remarkable victory is depicted in relief in four panels behind his gold-toned statue.
If you are looking for a quiet afternoon walk, with a commanding view of Seoul and the Han River, Haengjusanseong might be the answer.