Definite or indefinite article when describing a historical person

According to this question:
Definite and indefinite articles when introducing a person
one can use a definite, indefinite or no article at all when introducing a particular person. Which option is the most natural in case of a historical figure, like

ruins of a medieval castle built by the Polish king Casimir the Great

or

ruins of a medieval castle built by a Polish king Casimir the Great

and generally, are both definite and indefinite articles correct here? It will be used as a one-off sentence describing a picture on a stock photography site. Thanks in advance.

Answer

They are all grammatically correct but they have slightly different meanings. This is inevitably a bit subjective but I would say the difference is in which bit is the main information and which bit is the additional information:

ruins of a medieval castle built by the Polish king Casimir the Great
ruins of a medieval castle built by Casimir the Great (who was a Polish King)

We see this structure whenever we want to add extra information to a particular person

the famous George Washington
George Washington (who was famous)

This would not make sense if we said

a famous George Washington

as that would make George Washington and Casimir indefinite.

ruins of a medieval castle built by Polish king Casimir the Great

is a slightly clunkier way of saying the same thing.

On the other hand,

ruins of a medieval castle built by a Polish king Casimir the Great
ruins of a medieval castle built by a Polish King (whose name was Casimir the Great)

Here it is clear that the emphasis is on the fact that the builder was a Polish king (hence the indefinite article), to which we have added his name as an afterthought.