Old Great Square is the historic centre of Turku, close to the cathedral and the river, where the city's medieval, Swedish, and Finnish layers overlap. The square is framed by old civic buildings and has the kind of scale that makes Turku feel older and more intimate than Helsinki.
It is especially atmospheric during markets, festivals, and winter events, but even on a quiet day it anchors a good walk through the old city. Combine it with Turku Cathedral, the Aura River, and the nearby museums to understand why this was once Finland's most important town.