Kidnap victim Michelle Knight was treated like a “punching bag,” by accused kidnapper Ariel Castro, according to new details from the investigation.

While Castro sits in a jail cell charged with four counts of rape and three counts of kidnapping, the horror of what his captives endured for a decade is still emerging.

Castro, 52, abused all of the women, but Knight got it the worst, a family source told CNN.

He used her as his “main punching bag,” the source told the net