This is the 15996th most frequent Polish word.
tronu
"Tronu" means "of the throne" in English.
Here, 'tronu' is in genitive case, referring to the throne as the object which the king possesses or uses.
Król zasiadł na tronie.
The king sat on the throne.
In this sentence, 'tronu' again appears in the genitive case, indicating possession or association with the history being discussed.
Historia królewskiego tronu jest bardzo interesująca.
The history of the royal throne is very interesting.
The word 'tronu' in genitive form qualifies size, linking it specifically to the king's throne.