The most well-known building in the city is the 'Spaso-Preobrazhensky'. This monastery was originally founded in the 12th century and thus it, and its cathedral, are the oldest buildings in the city. The Transfiguration Cathedral itself, built in the year 1516, is the oldest detached building standing in the city. Typical for a Russian monastery of the Middle Ages, the Spaso-Preobrazhensky Monastery in Yaroslavl was not built to be no more than a place of worship, but also to be a citadel and kremlin in case, in times of war, there was a need for such a facility. This is still visible today as the monastery is surrounded by a thick 16th century, white-painted wall, complete with watchtowers and embattlements. Within these walls stand the magisterial churches, which, with their asymmetrically-ordered towers and beautifully decorated interiors, make for wonderful examples of traditional Russian sacral architecture. In addition to this there is a gatehouse church, with which the monastery's dungeons and treasury were connected. The monastery has long had a place in the history of Yaroslavl and continues, albeit nowadays as a museum, to play a significant role in the life of the city. It was largely thanks to the impregnability of the monastery that, during the time of the Troubles, the Russian peasants' army was able to defend the city and then go on to liberate Moscow from its Polish-Lithuanian occupiers. At the end of the 18th century, the oldest known text of the Tale of Igor's Campaign, the most renknowned work of Russian-language literature from the Middle Ages, was found in the library of the SPaso-Preobrazhensky Monastery. This masterpiece is now on display as a permanent exhibition within the monastery, along with other works of the age and an exhibition showing the conditions an author of the era would have lived in.