Gongsan Fortress is Gongju's headline sight, a Baekje-era hill fortress wrapped around a wooded ridge above the Geumgang River. Much of what you see is reconstructed or marked by foundations, but the setting is excellent: walls, gates, pavilions, and river views that make the old capital's defensive logic easy to understand.
The fortress is part of the Baekje Historic Areas UNESCO World Heritage Site. Allow time to walk the wall circuit rather than just stepping through the gate; the best moments are on the ridge where the river bends below.