Sea cliffs best climbed in the warm weather--late spring and early fall, as it can get hot and sweaty in the summer, but there is always dawn and dusk and great swimming in between. The rocks are off the beaten path. . . on the far north of the beach, accessed by a trail off the road behind the cliff-- there's a pull-out for parking). Keep left on the trail and after about 15-20 mnutes, you'll see a steep path down.
The rock can be sharp, and most of the routes are steep (and/or roofy), but as they say in real estate, location location location. this is the only place in Korea I've seen bioluminescents in the water, and there's beachfront camping about 2k down the road (or crowded campgrounds with amenities much closer).
Hagampo is accessible by public transport (busses from Taean, Chungcheongnam Province) but a car helps. In summer, there are busses from Cheonan. By car: from Taean (W of Seosan on route32) head N on 603, branching left on 634, following the signs to the beach (pretty much the end of the line, about 22k N of Taean)