He also began to reap the benefits of conversion: the clergy became an educated class upon whom the monarch could rely for diplomatic and administrative duties. By the thirteenth century, there were ambitious Swedish clerics in Paris and Bologna, and the first stone churches were appearing in Sweden. The most monumental is the early Gothic cathedral built at Uppsala. Meanwhile the nobility had come to form a military class, exempted from taxation on the understanding that they would defend the crown. In the country the standard of living was still low, although an increasing population stimulated new cultivation. The forests of Norrland were pushed back, more southern heathland turned into pasture, and crop rotation introduced. Noticeable, too, was the increasing German influence within Sweden as the Hansa traders spread. Their first merchants settled in Visby and, by the midthirteenth century, in Stockholm. The fourteenth century towards unity Magnus died in 1290, power shifting to a cabal of magnates led by Torgil Knutsson. As Marshal of Sweden, he pursued an energetic foreign policy, conquering western Karelia to gain control of the Gulf of Finland: and building the fortress at Viborg, lost only with the collapse of the Swedish Empire in the eighteenth century. Magnus' son Birger came of age in 1302 but soon quarrelled with his brothers Erik and Valdemar. They had Torgil Knutsson executed, then rounded on Birger, who was forced to divide up Sweden among the three of them. An unhappy arrangement, it lasted until 1317 when Birger had his brothers arrested and starved to death in prison an act that prompted a shocked nobility to rise against Birger and force his exile to Denmarl<. The Swedish nobles restored the principle of elective monarchy by calling on the three yearold Magnus (son of a Swedish duke and already declared Norwegian king) to take the Swedish crown. During his minority a treaty was concluded with Novgorod (1323) to fix the frontiers in eastern and northern Finland. This left virtually the whole of the Scandinavian peninsula (except the Danish provinces in the south) under one ruler. Yet Sweden was still anything but prosperous. The Black Death reached the country in 1350, wiping out whole parishes and killing perhaps a third of the population. Subsequent labour shortages and troubled estates meant that the nobility found it difficult to maintain their positions. German merchants had driven the Swedes from their most lucrative trade routes: even the copper and iron ore mining that began around this time in Bergslagen and Oalarna relied on German capital. Magnus soon ran into trouble, threatened further by the accession of Valdemar Atterdag to the Danish throne in 1340. Squabbles over sovereignty of the Danish provinces of SkSne and Blekinge led to Danish incursions into Sweden and, in 1361, Valdemar landed on Gotland and sacked Visby. The Gotlanders were massacred outside the city walls, refused refuge by the Hansa merchants.