The Russo-Swedish War of 1495-1497 was a result of an alliance between Grande Prince Ivan III of Moscow and Hans of Denmark, who was waging war against the Sture family of Sweden in the hope of regaining the Swedish throne. It is believed that Hans promised to concede some stretches of Finnish lands to the Muscovite prince, although he did not bother to honour the agreement after he had been crowned King of Sweden at the close of the war. Pursuant to the agreement, Ivan III sent Princes Daniil Shchenya and Vasily Shuisky to lay siege to the Swedish castle of Viborg. The siege lasted for three months and ended when a castellan set his supply of powder on fire, thus "scaring the Muscovites out of their wits", as the Swedish records say . The following year Russian generals Vasily Kosoy and Andrey Chelyadnin severely devastated Swedish Finland as far as Hämeenlinna . Sten Sture the Elder, who was then at Turku , was enraged at the news of the Muscovite expedition and sent Svante Nilsson with 2,000 men to take Ivangorod, a new fortress which Ivan III had built to protect Russian Ingria against Livonian Knights. The fortress was taken without difficulty, but — as it was obviously impossible to defend it for a considerable period of time — Svante Nilsson proposed to hand it over to the Knights, an offer which they declined. Thereupon the Swedes set the fortress ablaze and sailed home. After the Swedish throne fell to Hans of Denmark, hostilities were suspended until 1508, when Sweden and Moscow ratified a peace treaty for 60 years. Although the war yielded no tangible results to any of the belligerents, both countries corroborated the peace settlement in 1513 and 1524.
Question: Which fortress did the Swedish set on fire?
Answer: Ivangorod