In 1241 king Béla IV was trying to escape Mongol hordes and took an oath that he would offer up his yet to be born daughter to God if his family and country were saved from the enemies.
Thus, Princess Margaret born next year become a nun and in 1251 the king built a cloister and a church for the order she belonged to on the Island of Hares.
Today, this island is known as Margaret Island or Margit-sziget in Hungarian. It is a 2.5 km long and 500 meters wide island and recreational park in the middle of Danube and Budapest. In fact, it used to be three separate islands, but in the end of the 19th century they were connected and raised to control the flow of the Danube and flooding.