Koroistenniemi is the original site of ancient Turku. The large white cross marks the cradle of early medieval culture.
Before Turku settled in its present position, Koroistenniemi, located just off the Aura River, was a bustling centre of commerce, culture and religion. A wide variety of ruins remain as a testament to that age, including the stone foundation for a wooden church from the mid-1200s. Indeed, Finland's first extensive, modern archaeological excavations were done here in 1898-1902.
The National Board of Antiquities has named Koroinen one of Finland's most important Built Heritage sites. The site is marked by a large white memorial cross, which can be seen when arriving by train from Helsinki.
http://www.visitturku.fi/en/ancient-ruins-of-koroinen_en