Place prepositions follow logic. For an enclosed area or city — 'She lives ___ Seoul' — which preposition fits?

"In" is the correct preposition for cities and other enclosed or bounded areas, which is why English speakers say "in Seoul" rather than "on" or "at" Seoul. Prepositions of place in English generally follow a rough logic of enclosure versus surface versus specific point, and a city counts as a bounded area you exist within.

"On" typically describes surfaces or specific streets, as in "on Main Street," while "at" pins down a precise point, as in "at the corner" or "at 5th Avenue." Neither fits the broader, enclosing sense that a whole city requires.

This in/on/at system extends in layers: you might live "in" a country, "in" a city, "on" a street, and "at" a specific address, moving from the broadest enclosure down to the most precise point — a nested logic that native speakers absorb intuitively but learners often have to memorize explicitly.