Casimir III is the only Polish king to ever receive the title of "the Great". His reign was a time of prosperity and growth for the Polish kingdom. He focused on the economy and strengthening of the country and not on wars and expansion. He kept peace with the Teutonic Order during his reign, for he knew that Poland was weak and would not survive the confrontation with the knights. Because of that, he focused his attention on the south-eastern borders of the country and expanded there. In 1340, he conquered Red Ruthenia (Lviv along with it) from Hungary. Various tradelaws created by him made it easier for Polish cities to gather more wealth. Cracow and Lviv gained in importance during his reign.
Curiosities
There is a proverb about the king: "Zastał Polskę drewnianą, a zostawił murowaną" (When he ascended Poland was wooden, when he left us, it was from stone) – he had raised many castles during his reign.
The old Jewish part of Cracow is called "Kazimierz". It was one of the many cities established by the king.