Castro, who for 49 years headed a government frequently accused of mistreating prisoners by human rights groups, said Cuba had never tortured captured spies.
He was referring to Gererdo Hernandez, one of five men captured by the US in 1998 and accused of being members of a Cuban espionage ring tasked with gathering intelligence on members of the Cuban exile community in Florida.
Prosecutors said the men had been involved in the shooting down by Cuban jets of two planes dropping pro-democracy leaflets on Cuba. They received sentences ranging from 15 years to life in prison.
Cuban agent ill
Castro alleges that Hernandez has been placed in a tiny cell with another prisoner.
"There are two men in a space one metre in width," Castro said. "He's not only in a high-security prison, which is already a deep hole, he's in a hole inside the hole."
Castro said Hernandez was ill, too. "He's sick, he could have a bacteria," Castro said. "He's a person who needs medical assistance." Hernandez, the leader of the five agents, is serving two life sentences for his role in the plot.
The former Cuban president's comments came as the US demanded the release of one of its citizens that Cuba has imprisoned for spying.
Alan Gross was jailed in Havana last December on suspicion of espionage, but has not been officially charged. US officials say that his health is suffering and on Friday called for his detention to end.
Castro has made a series of public appearances in recent weeks, emerging from four years out of the limelight. He had stopped making public appearances after undergoing serious surgery, and handed control of the Caribbean island to his brother Raul.
Raul offically became president in 2008, when Castro tendered his formal resignation.