Busses run from all nearby subway stops to Ui-dong. . . Suyu is good as any and the posts for Insu and the Insu boulders list the bus numbers. . . 109 and 151 --both blue-- go there from across town: Seoul Station, Sinyongsan, City Hall, Cheongdeok Palace.
From the stop head up the road to across from where the blue and green busses park. . . you can catch the shuttle to the park/trailhead. If the line is daunting, catch a taxi. . . you'll regret walking when it costs a couple bucks tops.
The ranger hut has topos if you don't, but many of the climbing shops near 02 World have the free Insu route guide. . . Sunday they'll be a billion people there. . . get up early or wait in line! Lots of groups go there Sat PM and camp, getting on the routes early. . . often in groups of 4 or more. . . Definitely wear a helmet-- more than one person has been killed here from rock (or other stuff) fall. Some commemorative plaques remember some of them.
is probably the best online route guide. It's in korean, but pretty easy to follow. . . 크랙=crack (interpretation varies), 슬랩=slab, 페이스=face, 위주=chimney, 오버행=overhang, 인공등=aid, 볼트길=bolt ladder, 퀵드로= quickdraw (usually folowed by the #) 프렌드= friends (camalots will work
). You can click on individual routes for more specifics. Print out the route(s) you want and go! (You might have a few back-ups in case the routes are backed up!!)