Havana, Cuba (CNN) -- Former Cuban leader Fidel Castro lashed out at U.S. President Barack Obama on Thursday for demanding changes on the island in order to improve bilateral relations and referred to him as "stupid."

On Wednesday, Obama told Hispanic journalists that he would be willing to change the country's tough line on Cuba "when we start seeing a serious intention on the part of the Cuban government to provide liberty for its people."

On Thursday, Castro responded with sarcasm in a written essay published in Cuban state media.

"How nice! How intelligent! So much generosity has failed to let him understand that after 50 years of blockade and crimes against our country, they haven't been able to bow our people."

He went on to say many things would change in Cuba, but only thanks to Cubans' own efforts and "despite" the United States. "Perhaps that empire will collapse first," he added.

He also slammed a recent ruling by a U.S. judge against a Cuban agent, but he said it was to be expected.

"Otherwise, the empire would cease to be the empire and Obama would cease to be stupid."