The castle in Olsztyn was under the Kingdom of Poland from the mid-15th century and Catholic Church officials of the Warmian Chapter were responsible for administrating it.

One of them was Nicolaus Copernicus, who in 1516-1521 administered the chapter’s property, including its nearby lands and the castle in Olsztyn. He was a canon, that is a member of the Catholic clergy, who professed his first vows, including the vows of chastity. But he wasn't a priest because he was never ordained.

Castle under Copernicus

During his administration of the Olsztyn Castle, another war with the Teutonic Order broke out. In 1519 he participated in unsuccessful negotiations with Albert of Prussia, the Grand Master of the Teutonic Knights. In January 1520, Copernicus was in Frombork when the Teutonic Knights attacked and torched the town.

As he began preparations to defend the castle against the enemy, the astronomer sent a letter asking the Polish King Sigismund the Old for military help.

Even though the Teutonic Order seized the letter, the insightful king sent Copernicus 100 armed men. Copernicus knew all too well that the men would not be enough to defend the castle. Still, he made solid preparations to fend off the attack. In fact he was so successful that in 1520, after one attempt, the enemy withdrew from its plan to conquer the castle.

Treatise on Coin

When he administered the castle, King Sigismund I the Old commissioned Copernicus to write a treatise on money, the result of which was "Monetae cudendae ratio" also called Treatise on Coin. In his paper, Copernicus proposed introduction of a unified monetary system and incorporated formulated the economic law which states that “bad money drives out good”.

He worked on his treatise on and off from 1517 to 1522. In 1528 he expanded his work, laying the foundations for a monetary policy. The title of the new document was “On the Minting of Coin.”

During his stay at the Olsztyn Castle, Copernicus also conducted astronomical observations. He was particularly interested in planets and issues related to vernal and autumnal equinox. To follow the equinox better, he constructed a special astronomical table.

Today the table is near the entrance to a castle chamber, which Copernicus once occupied and which houses an exhibition dedicated to the astronomer.