The boulder fortress of the Solovetsky Monastery, erected at the turn of the 16th-17th centuries, is an unsurpassed example of Russian defense architecture of the late Middle Ages. For two centuries, it was the center of defense of the Russian North, but at the same time defended the holy monastery, being one of the most important centers of the sovereign pilgrimage in Russia. This monograph, based primarily on archival and archaeological sources with the addition of architectural materials, introduces numerous new information about its history and architecture.