Köln is a big city close to other big cities, that is almost grown togheter with each other, if they ever will become one city, then it would be the biggest in Germany. Köln used to be the capital for long time atrás too. Now it's Berlin.
This video shows the history of Germany and Austria from the foundation of East Francia to today. Occupied territories aren't shown until 1789 and all the states inside the Holy Roman Empire aren't shown, but only a few.