Kastellinlinna in Pattijoki is the the largest of the jätinkirkko ("Giant’s Church") formations in Finland.
It is a stone formations in the shape of an oval, built around 3,000-1,600 B.C. It's not known for sure for which purpose these stone structures were built. Most probably they were campsites used by seal hunters. Though others suggest that they might have been used as ritual sites.