"It was known as the Polish city of Królewiec from 1454 and a fief of Poland from 1456 to 1657."
You might be confusing the Polish DUCHY of Prussia (which this was a part) with the later separate KINGDOM of Prussia?
"It was known as the Polish city of Królewiec from 1454 and a fief of Poland from 1456 to 1657."
You might be confusing the Polish DUCHY of Prussia (which this was a part) with the later separate KINGDOM of Prussia?