The monastery gradually grew in size and reputation. The 16th century was the time of its flourishing, when under Abbot Cornelius the Annunciation and St. Nicholas Churches were erected as well as the stone belfry and the monastery was fortified around by a stone wall with bastions, and the brethren numbered up to 200. Abbot Cornelius collected a rich library, established chronicling and founded an icon-painting workshop. The monastery became an important missionary center and stronghold of Orthodoxy on the western border of Russia. Tradition has it that, slandered by envious people, Abbot Cornelius was beheaded by Tsar Ivan the Terrible on the 20th of February, 1570 and now numbers among the hosts of holy martyrs.In the 16th and 17th centuries the monastery suffered from numerous attacks by the "Latins" — Polish and Lithuanian troops, as well as by the Swedish and German soldiers. The most serious attack was lead by the Polish King Stephan Batory in the autumn of 1581.