ENEven though disputes between the different religious confessions were frequent, it is believed that open conflicts between the opposing sides were nevertheless quite rare in 16th-century Vilnius and nowhere near as common as those that took place in Poland’s larger cities where the Protestant movement was weaker. Four religious turmoils in the capital of the Grand Duchy of Lithuania have received some attention: those in 1581,1588, 1591 and 1598. The article presents the popular turmoil of 1599 that broke out between Catholics and Protestants. The religious atmosphere in the city was influenced by the promotion of Counter-Reformist policies that successfully developed after the 1596 Union of Brest. The unification of Churches opened up new opportunities for the Protestants who were considered as allies by the Orthodox believers who did not accept the union (the so-called Dis-Uniates). Vilnius became one of the most important collaboration centres for representatives of these confessions, as well as a flashpoint of tense religious opposition. The first outburst of intolerance did not take long to develop. Before April 25 in 1598, students of the Vilnius Academy attacked the Orthodox Church of the Holy Spirit, followed by this brotherhood’s school the next day. The perpetrators of this attack were left unpunished. This event urged Orthodox and Protestant believers to engage in closer collaboration ahead of a general conference. The planned date of the conference in Vilnius, May 9,1599, was specially selected to coincide with the beginning of a session of the Lithuanian Tribunal and was kept secret for a long time. It was hoped that Catholics would relate the strong Protestant presence in Vilnius with the latter’s court matters. So from the beginning of 1599 the Protestants prepared to welcome a large number of non-Catholics in Vilnius (both Protestants and Dis-Uniates).On April 12, 1599 (i.e. Easter Sunday evening), during mass in the Cathedral in Vilnius’ Lower Castle an argument erupted between Protestants and Catholics which later spilled out beyond the Cathedral itself. Once fighting erupted security guards closed the castle’s gates, which only faned the conflict’s flames further, as the bishop and the Catholics could not leave the castle and go into the city, and upon hearing about the conflict that had erupted a crowd of Catholics descended on the castle. The castle’s Rittmeister rejected the Samogitian bishop’s request to open the castle’s gates for fear of putting the ruler’s treasury, privileges and other treasures kept in the castle at risk, plus he also wanted to find out who was to blame for the insurrection. It was suggested that the bishop and his followers head for the western gate near the estate of the Vilnius voivode. However, the crowd refused and only departed the Cathedral at daybreak, once the danger had passed and the castle’s gates were again opened. Failing to enter the castle itself, the crowd stormed the Evangelical Reformist Church. During the attack a preacher was killed on the street near the gates and a number of Evangelists were injured. Fearing their church would be set ablaze, the Protestants protected it throughout the night. There are two different versions of these events. The Protestants accused the Catholics of beating up their Bachelor Nestor Dziewiąty, which provoked the insurrection in the castle. They closed the Vilnius Lower Castle’s gates for safety, as, purportedly, the townsfolk that had been invited by the Catholics started surging into the castle. Once the attack on the castle failed, the crowd turned upon the Evangelical Reformist Church. The Protestants lost a preacher and many people were injured. [...].Historical sources dealing with the events of 1599 give us some new, as yet unknown information about the Vilnius Lower Castle. The castle’s security was in the hands of hajduk infantrymen, as per the example of the Hungarian army. In case of danger, the security of the castle was the concern of the Vilnius vicegerent and his deputy, the castle’s Rittmeister being subordinate to them. Our knowledge about the castle’s Rittmeister’s dwelling and the construction of the southern castle gates from the 16th century has been broadened (earlier information hinted at the existence of a Rittmeister’s dwelling only since the 17th century). The sources confirm that in the 16th-17th centuries the castle’s defensive system demanded renovation, and that the courts of the nobility, the Tribunal and the Castle court, already traditionally assembled within the castle grounds. Sources revealing the most information about the religious turmoil of 1599 are publicized in the article.