Kingdom

>Kingdom.
>Peninsula
>Mostly plains
>Lots of rivers
>Small mountain chain which is the border of the kingdom.

How would the economy look like?

Is there a mainland nearby?

Probably mostly fishing and light seasonal agriculture or forestry. It really depends on what the climate is like and what they can trade with their neighbors.

Ok, that really depends.

What sort of climate do they have? What are the growing seasons? Is it marshy? What sort of mineral wealthy is around? What kind of neighbors do they have? Where are they geographically in relation to other powers.

You have given us not nearly enough information to even begin postulating.

Depends on the size of the kingdom. A large one may be based on feudal agriculture, while a small nation could become rich through maritime trading

Germany-Spain/Denmark/Sweden...your choice.

Mostly agrarian with some mining operations in the mountain chain.

France or Germany depending on the distribution of the rivers.

Farming and trade

The most important element: Are these rivers navigable?

Also, how fertile is the ground?

Salt mining as punishment for political prisoners