One is Krak's Mound in Kraków, believed to date from around BC 500. Krak was the name attributed to the legendary king of the forerunners of today's Poles, known as Lusatians. There is disagreement as to their ethnicity, believed by some to have been proto-Germanic.
A glimpse of Krak's Mound is found at
pl.wikipedia.org/wiki/Kopiec_Krakusa