There's no real Chinatown in Seoul proper, as far as I know, but there's a cluster of Chinese restaurants near the Chinese Embassy in Myeong-Dong, not far from where I was staying. I went out one sub-freezing evening in search of some Korean Chinese food and went into the restaurant that was the most crowded.
Korean Chinese food is a distinct cuisine. While many dishes are based on Northern Chinese food, they've been modified significantly. One of the most famous noodle dishes is jjajiang myun, a variant on chachiang mein, which on English-language menus is also known as noodles in hot brown meat sauce, or noodles with meat sauce Peking style. But the Korean version has a thicker, sweeter sauce, and usually seafood instead of meat. I find the Korean Chinese version too heavy in general.
Another noodle dish, which seems to have been invented by Chinese in Korea, is jjam ppong. It's a very spicy seafood noodle soup (shown at top) that could be mistaken by the casual observer for linguine with mussels--but that ain't no tomato sauce, that's a broth full of crushed red chili peppers. The version I had at this place was the best I've ever tasted, though I prefer my noodles chewier than they served them.
I also ordered the ma po tofu over rice. It was fairly close to a true Chinese version, except that the spice was purely from red chilies, not Sichuan peppercorns, so there was no tongue tingle. It was certainly better than those cornstarch-thickened brown monstrosities that American Chinese restaurants call bean curd Szechuan style.
I can't tell you the name of the restaurant because the only English on the sign said "Chinese Restaurant." The Chinese owner or manager knew enough English to warn, "Both dishes very spicy!"
"That's OK," I said, figuring "Bring it on!" might not be as easily understood.