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.