They hit the front four minutes before the break at La Romareda when Castro latched onto a through ball from Jorge Molina and blasted a right-foot shot past Roberto Gago in the Zaragoza goal.
The 30-year-old former Spain under-21 international scored a second - his ninth league goal for the season - in the 68th minute with another tidy shot from the centre of the box to put the result beyond doubt.
