First you will need to sketch the doorway.
Decide if your door is going to be single or double. Sketch the planks on the door, keeping the width fairly even.
If you create an arch on the door, then you should try putting a keystone at the top centre of the arch. Using the first stone as the start, add more stones to the archway.
When drawing steps, make the sides slant down in one-point perspective and the lines on the tops of the steps will meet at a vanishing point on the unseen horizon line inside the castle.
Now sketch the door with the hinges in place from the outside. You do not need to sketch every block in a stone wall or all of the bricks in a wall to give the impression of a stone or brick wall. Just sketch a few to show the texture and then fill in the rest. This will automatically give an idea to the viewer that the wall is present.
Sketch wood grain patterns on each of the planks in the door with a lighter brown colour and then the lines between the planks, while putting knots and changing direction from plank to plank. Most planks warp a lot less if the grain in each plank is set at opposing angles per plank.
Fill in the door with a light tan over the wood grain texture, then shade the archway and some of the detail stones with a very light gray. Shade the front of the steps and not their tops, as shading these will lighten the effect of the stairs.