Step1
Sketch of arched double medieval door with arch, keystone and steps
Sketch the doorway. Often, medieval doors were arched rather than flat across the lintel, but sometimes they had a heavy wooden frame flat across the lintel. Decide if your door is going to be single or double.
Sketch the planks in on the door lightly, keeping the width fairly even. Medieval doors were thick and made by joining heavy planks together with tongue and groove or other methods, solid wood rather than hollow core like today's doors. Church doors are good models for medieval doors, on old fashioned Gothic churches.
If you arch the door, be sure to put the keystone in the center of the top of the arch. It would fall down if a join between two stones came at the top of an arch, there is always a center stone with angled sides going in both directions. Sketch that first and draw the other stones of the archway from that one.
When drawing steps, make the sides slant down in one-point perspective, the lines of the tops of the steps would meet at a vanishing point on the unseen horizon line inside the castle.
Step2
Diagram of simple medieval hinge showing construction and how the parts fit together for modelers
Medieval hinges were leaf-shaped, pounded out of iron or bronze. They folded around and were attached to thick wooden doors from both sides with nails or bolts. Here's a sketch of just one hinge before it's attached to the door.
There would be two, three, four or five of them per door on the sides.
Also on the inside, the door will usually have brackets to drop a big post into for barring the door. The post might be on its own hinge and pull up with a pulley, or need to be manually slid into place like a deadlock post.
The diagram shows how the hinge looks in detail, how it fits with the one that gets bolted to the wall (which does not usually get two sides, only the one that attaches to the wall and then welded back to itself around the curl), and the hinge pin which gets a bolt on the end to keep it from popping out once in place. The number of bands may vary, but blacksmiths didn't always get complicated.
Step3
Sketch inked in gray, black and brown
I simplified the hinge, but it probably had a gap in it for multiple bands to intersect each other from a mount on the wall of the castle.
Let's sketch the door with the hinges in place from the outside in felt tip pens,
You don't need to sketch every block in a stone wall or every brick in a brick 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.
I've drawn the hinges in black because they're black iron, the lines of the planks in dark brown and the stonework outlines in gray.
Step4
Medieval hinges in place on a double arched door with wood grain texture and shadowing
Sketch wood grain patterns on each of the planks in the door with a lighter brown than the lines between the planks, putting knots in and changing direction from plank to plank. A carpenter once explained to me that planks warp less if the grain in each plank is set at opposing angles per plank, and this was probably well known in the middle ages.
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.