As the legend goes, Prince Popiel ІІ was a cruel and corrupt ruler who cared only for wine, women, and song. He was greatly influenced by his wife, a beautiful but power-hungry German princess. Because of Popiel's misrule and his failure to defend the land from marauding Vikings, his twelve uncles conspired to depose him; however, at his wife's instigation, he had them all poisoned during a feast (she might have done it herself). Instead of cremating their bodies, as was the custom, he had them cast into Lake Gopło.
When the commoners saw what Popiel ІІ and his wife had done, they rebelled. The couple took refuge in a tower near the lake. As the story goes, a throng of mice and rats (which had been feeding on the unburnt bodies of Popiel's uncles) rushed into the tower, chewed through the walls, and devoured Popiel and his wife alive. Prince Popiel was succeeded by Piast Kołodziej and Siemowit.
On the shore of Lake Gopło stands a medieval tower, nicknamed the Mouse Tower; however, it cannot be the site of the events described in the legend as it was erected some 500 years thereafter.
^Popiel's wife is sometimes called Ryksa (Hilderyka, or Brunhilda); however, her name was never specified by medieval chroniclers (see: Historya narodu polskiego, Volumes 3-4. By Adam Naruszewicz, 1836. Chapter "Podług rękopisma Puławskiego")