Hwaseong Fortress, the place I fell in love with. If you want to experience what this World Heritage Site has to offer, you have to stay there for at least two days and one night (I keep coming back there for three consecutive days and two consecutive nights). Experience all the performances from 24 Martial Arts performance to Security Guard Ceremony (happens from Tueday-Sunday all year round). Also, walking around the walls from inside the fortress and then outside, daylight and nighttime, is a must.
To go inside the palace, it only takes KW1,500. Hotels around the fortress is about KW40,000/night.