 The most important sources are Novgorod First Chronicle, Primary Chronicle and Eric Chronicles.Fortifications are known from Finland already from the Stone Age onwards.In Yli-Ii by the Iijoki river is located the Kierikki Stone Age fortress, which was built on piles and fortified with palisade.Also the approximately 40 Giant's Churches from the Neolithic period (3500–2000 BCE) found from the northwest coast of Finland may have served as fortifications.