The home side took the lead in the opening stages of the first half following a free kick conceded by Castilla in the 14th minute.
Following the set piece, the ball fell kindly to Marchena who was free in the centre of the box, allowing the defender to crash the ball into the net.
The team shared an equal amount of possession in the first half as Deportivo protected their lead, with Castilla finding it hard to break them down.
The visitors were presented with an even harder task in the second half as they lost Leandro Cabrera to a red card following an inexcusable foul straight after the break.
Coruna made their extra man count and finally rounded the game off in the closing stages with a second goal, this time Fernandez let fly from inside the box.