After Gaul and the territory up to the Rhine had been incorporated into the Roman Empire in the first century BC, Jülich was founded as the Roman station (statio) of Juliacum one day's journey from the starting point of the main Roman road in Cologne, at a narrowing of the valley of the River Rur, which at that time was difficult to pass. Finds of votive stones, potter's kilns, hypocausts etc. testify to the rapid growth of a settlement.

Jülich is first mentioned with a reliable date in AD 356 on the occasion of a skirmish between the troops of the later Emperor Julian the Apostate and 600 Frankish warriors. Such forays by the Germanic tribes led in about 310 to the construction of the first Roman fortification, which survived the decline of the Roman Empire in the middle of the 5th century and gave rise to a Frankish settlement. This became the residence of the counts of Jülich, originally royal officials, who were able to make their office hereditary after about 900. In 927 Jülich was mentioned in a document as a "stronghold".

It is impossible to understand the history of the town - Jülich was the principal town of the district during the 5th-9th century - without a glance at the politics of the counts (dukes from 1356). Their politics were never simply confined to the district of Jülich itself; depending on the situation, warlike expeditions, financial means and also marriage contracts were used to extend their sphere of influence.The first count of Jülich known by name, Gottfried, owed his lands to his father's (and other Lothringian nobles) victory over King Zwentibold in 900 and probably also to his father's marriage to the widowed queen. Another important marriage in 1177 added further extensive lands in the Eifel Hills with a considerable income (e.g. from mining). Many German kings took up loans from the counts of Jülich (amongst other things for coronation expenses in Aachen) and had to pledge imperial property as security (thus for example the imperial town of Düren in 1246).

However, the counts' campaigns were not always so successful, Jülich was destroyed twice when the counts ended up on the "wrong" side in disputes about the throne: in 1114 by Emperor Henry V and in 1214 by the King and later Emperor Frederick II.

The struggles against the supremacy of the archbishops of Cologne lasted for almost a century. In 1234 Count William IV raised Jülich to the status of a town without considering the archbishop's rights. This resulted in the capture and destruction of Jülich by Archbishop Conrad in 1239. Two archbishops of Cologne were captured in later conflicts and incarcerated in the castle dungeon at Nideggen in order to make them yield to the will of the counts of Jülich. A final crisis arose in 1278 when William IV was struck dead during unrest in Aachen where he was acting on behalf of King Rudolph in taxation matters. Jülich was then immediately destroyed once again by Archbishop Siegfried.

However, Jülich's independence was finally guaranteed by Count Walram's victory at Worringen in 1288. Shortly after this in about 1300, the town was given a new wall, of which the Rur Gate (the so-called Hexenturm - witches tower) remains as the oldest building in Jülich. This town wall protected Jülich for 250 years, during which period the sphere of influence of the counts and dukes increased continuously; the possessions of Berg, Ravensberg, Cleves and Mark, for a time Geldern as well, were added by marriage and succession. However, the ruling family later seldom resided in Jülich, their preferred residences were in Nideggen, Hambach, Kaster, Nijmegen, Düsseldorf and Cleves.

The town was ravaged by major fires in 1473, 1512 and 1547, so that a complete reconstruction was planned. Furthermore, Duke William V (the Rich, *1516 Cleve, a 1592 Düsseldorf) had lost a war against the Emperor Charles V in 1543 (over the Duchy of Geldern) during the course of which the medieval town wall had proved to be obsolete. Duke William engaged the Italian military and civil architect Alessandro Pasqualini (*1493 Bologna, † 1559 Bielefeld) to undertake the rebuilding of the town and construction of a fortress with the most modern means available at that time. So Pasqualini built the Renaissance town and fortress of Jülich, the conception of which with the citadel fortress and the palace, the town fortress and the ideal town layout was to survive intact for the next 300 years in spite of all political vicissitudes (the ruling house died out in 1609 and the territory was divided up between the heirs).

The fortress was enlarged towards the end of the 17th century and also later extended by adding outworks. The forts built by Napoleon I on the west bank of the Rur and on the Merscher Höhe (an elevation to the east of the town - this fort was not completed) also represented a considerable reinforcement. By 1860 the development of military engineering had reached such an advanced stage that the Jülich fortifications no longer had any significance. The bastions, of which only sections remain, and walls of the town fortress were then razed to the ground as part of a siege exercise by the Prussian army using artillery with the recently invented rifled barrel. The citadel remained almost completely intact.