Citibank ATMs are everywhere in Seoul, much more so than in Tokyo, so you shouldn't have a problem getting cash with a U.S. card.

Well, actually, I did. I went to a branch of Citibank to use their indoor ATM, and the machine did all the right things, including spitting out a receipt and returning my card, but no cash came out.

I looked around frantically and found only a reception desk. Fortunately, one of the receptionists spoke enough English and gesture language to figure out what had happened, so she called a higher-up, who went behind the wall of machines, fixed the jam, and handed me the bills that were on my receipt.

Fortunately, that was the only problem on the entire trip.