Being a member of the influential Velyaminov clan of boyars, Kirill relinquished the office of father superior of the greatest cloister in medieval Moscow, the Simonov monastery. His close ties with the ruling elite can be convincingly demonstrated by his letters to sons of Dmitri Donskoi. It seems that the Muscovite rulers regarded Kirill's monastery as an important strategic point, both for Northern trade and in their struggle with the Novgorod Republic. By 1427, when Kirill died, the prince of Belozersk-Mozhaisk (subject to the Grand Prince of Moscow) was the monastery's patron, and the monastery was administratively subordinate to the Archbishop of Rostov. Under Hegumen Trifon (1434/5–1447/8), social and administrative reforms were undertaken, including the adoption of an Athonite cenobitic rule. A Byzantine-style secondary school was established at which translations of textbooks on grammar, semantics, geography, and history were used. A lasting legacy of the school were bibliographical studies, exemplified by the elder Yefrosin, and text-critical studies, exemplified by Nil Sorsky (1433–1508). Nil also founded a skete on the Sora River near the monastery.