Christianity was brought to Estonia through the northern crusades, and brought about huge changes in the society, culture and architecture of Estonia. The influences came mainly from German-speaking areas and Scandinavia. The new religion prompted the erection of churches throughout present-day Estonia, beginning in the 13th century. The first churches were probably wooden; of these none survive. The earliest churches still surviving were strongly built and fortress-like; Valjala church on Saaremaa is one example. The master masons and sculptors who played a significant role in the early church architecture came mainly from Gotland. Later, as Christianity became more firmly rooted, larger and more elaborate churches were built, especially in northern Estonia during the 15th century. The medieval churches in the old town of Tallinn, like the Cathedral, St. Olaf's church (the tallest building in Europe in the 16th century) or the church of the Holy Ghost, still testify to a highly developed Gothic style of architecture. Outside of Tallinn, many churches were damaged during frequent wars, especially the Livonian War and the Great Northern War, and negligence during the Soviet occupation caused much destruction among the churches of Estonia, thus few early churches survive unaltered to this day. In later years however intense restoration work has been carried out, and especially on Saaremaa and in northern Estonia there are still interesting examples of relatively well-preserved medieval churches.