In order to pursue their ambition, and to designate a seat of their principality, the politically thriving Counts of Celje, elevated to the ranks of princes, needed a town with all the administrative bodies of the time, as well as a mayor. This surely spurred the development of Celje, although the settlement was "merely" a military and administrative centre of the rapidly developing Celje landed estate until the early 15th century. The presence of Jewish bankers, revived construction industry, crafts, and trade fuelled Celje's importance and reputation before 1436 when it is first mentioned as a town in historical documents, although it formally did not yet have such status at the time. Thus, the document dated April 11, 1451, by which Friderik II, upon request "by all residents of our Celje", granted Celje all town privileges of the Styrian towns, was merely an acknowledgement of the already existing status.
In this period, the town's development picked up the pace. Town walls were completed in 1473, and some notable buildings were constructed, including the Lower Castle, one of the most prominent renaissance palaces in Central Europe at the time. The downfall of the Counts of Celje in 1456 ended a major, perhaps the most important period of the Celje history. The age of renaissance humanism which had been finding its way to Slovenia at the time, had a great impact on Celje. The princely capital was then an important and virtually the only center of renaissance humanism in Slovenia. Sadly, however, the new culture started to fade after the death of the last member of the Celje dynasty.