Modern Korean food is evolving. Compared to 100 years ago, modern kimchi recipes use 100 (!!!) times as much chilli - but the modern chilli powder/flakes used now is much milder, and the older kimchi may well have been spicier. Compared to 50 years ago, the cuisine has become much meatier - annual meat consumption having risen from 5kg per person to over 70kg (not including seafood). The amount of sugar in Korean cooking is enormously higher today than in the past. But more or less, the Korean flavour base was, and still largely is (with the addition of lots of sugar), garlic, sesame (oil and seeds), soy sauce and bean paste, green onion, and chilli. The only Columbian Exchange member of this spectrum is the chilli. Before the chilli? The "spicy" spices would have been black pepper (imported), Sichuan pepper, and ginger.

Ginger has been used in Korea for a long time (and is also a long-used ingredient in traditional Manchurian cooking). The main Korean species of Sichuan pepper (Zanthoxylum piperitum) is the same species grown in Japan, but is closely related to the usual Chinese (and Tibetan) species (Z. simulans, Z. bungeanum, and Z. schinifolium (this last species also grows in Korea and Japan)). The main Korean name is chopi (초피), and similar in dialects (and sanshō (山椒) in Japan). Not chilli-hot as such, but spicy and numbing. Chopi appears to have largely fallen out of use due to replacement by chilli, but has survived in southern Korean cooking and in Korean traditional medicine. The other prominent Korean spice, if we are to classify garlic as a spice, is garlic. Garlic might have preceded humans into Korea, spreading into the peninsula as the ice sheets receded - the centrality of garlic in the Dangun myth, as the sacred food eaten by Dangun's mother to transform from bear to human, may well reflect the centrality of garlic in Korean cuisine from its very beginning.

The oldest surviving Korean cookbooks show us what pre-Columbian Korean cooking looked like. Perhaps the most notable early cookbook is Eumsik dimibang, written in Korean (in Hangul), by a woman (and presumably for women), in about 1670. While this is after the chilli was introduced into Korea (it appears in Korean literature as early as 1613), chillies don't appear in Eumsik dimibang. As an example of a recipe from this book, here is (in English) the recipe for Japchae:

Prepare raw chopped cucumber, radish, Cham mushroom, manna lichen, pyogo mushroom, and mung beans plant. Boil and tear chinly balloon flower, clover, dried slices of young pumpkin, shepherd's purse, dropwort, spring onion, edible shoots of a fatsia, bracken, spinach, gourd, egg apple and pheasant meat. If there is no ginger, use dried ginger or vinegar-pickled ginger. Season with pepper, sesame oil, dark soy sauce, flour and chop all sorts of ingredients to thin pieces. Fry each with oiled soy sauce and put on a wide bowl by mixing some together or separating. Sprinkle an adequate amount of chopped pheasant meat on it and cheoncho, pepper and ginger as well. Filter thick paste to have moderate taste. Add sesame oil and flour. If the taste is good, boil it over once. Do not make the source thick. As for wax gourd, parboil quickly when it is fresh. For coloring, use ballon flower and cockscomb for red or wild grapes. There is no need to get all those materials above. Just do with the materials that you can get easily.

This translation is from https://artsandculture.google.com/exhibit/AQXUFTc6 which also has lovely photos of reconstructions of some of the dishes, and further recipes. I don't have the original Korean text at hand, so can't cay what is in the text that is translated as "pepper" above. However, early appearances of "pepper" in the Korean sources written in classical Chinese use 椒 ("jiāo" in Mandarin), which is Sichuan pepper. So the Eumsik dimibang will be chopi (Sichuan pepper) or possibly black pepper.