Today's Miskolc came into existence with the unification of the historical Miskolc (today's city centre), the nearby Diósgyőr and several smaller towns between 1945–1950. United, the busy merchant town Miskolc, and Diósgyőr, which had been a favourite royal residence in the Middle Ages, formed the largest city in the region (with a population peak over 200,000 in the 1980s) and was developed into an important heavy industrial centre. With the industry's decline in the 1990s, Miskolc is now emphasizing its cultural heritage in hope of becoming a tourist attraction.
It was one of the favorite castles of King Louis I, who added extensively to the building. Two historical events that took place in the Castle of Diósgyőr during his reign were the granting of a coat of arms to Kassa – the first time in history that a city received a coat of arms –, and the signing of a peace treaty between the Kingdom of Hungary and Venice. Both events are commemorated by full-size wax figures of the king and other persons in the wax museum of the castle.
In later centuries the castle was a favourite holiday residence of queens, with the first of them being Queen Regnant Mary, the daughter of Louis. Incidentally, the last queen to own the castle was also a Mary – the wife of Louis II.
In the Middle Ages Miskolc, a thriving merchant town, was the most important of the towns and villages belonging to the estate centered around the castle. Later, as Diósgyőr's importance decreased, Miskolc outgrew it by far, and by the time the two towns were united in 1945, it was three times larger.