Desde el mirador, quiero hacer fotos del castillo al atardecer.

Desde means from or starting from a place. Here it tells you the viewpoint or location from which the speaker wants to take the photos.

So:

• desde el mirador = from the viewpoint / from the lookout point

It is different from en el mirador, which would mean at the viewpoint or in the viewpoint area.
Using desde emphasizes the point of view: the photos will be taken from there.

A mirador is a viewpoint, lookout point, or scenic overlook—a place where people go to enjoy the view.

In Spain, this is a very common word, especially in towns, mountains, or tourist areas.

Examples:

• Subimos al mirador. = We went up to the viewpoint.
• Hay un mirador cerca del castillo. = There is a viewpoint near the castle.

In Spain, hacer fotos is very natural and common. It literally means to make/take photos, but in English we translate it as to take photos.

Other possibilities exist:

• hacer fotos — very common in Spain
• sacar fotos — also used in many places
• tomar fotos — more common in much of Latin America than in Spain

So for Peninsular Spanish, quiero hacer fotos sounds very natural.

Yes. Quiero sacar fotos del castillo al atardecer is also correct and natural.

The difference is mostly regional or stylistic:

• In Spain, hacer fotos is especially common.
• Sacar fotos is also understood and used.
• Tomar fotos is less typically Spanish-from-Spain, though people will understand it.

So if you are learning Spanish from Spain, hacer fotos is a very good choice.

Because de + el contracts to del.

So:

• de el castillo → del castillo

This is a standard contraction in Spanish, just like:

• a + el → al

Examples:

• la puerta del castillo = the door of the castle
• voy al castillo = I’m going to the castle

One important exception: you do not contract if el is part of a proper name.

Example:

• de El Escorial — not del Escorial, if El is part of the name

Because del castillo means of the castle or showing the castle.

The photos are about / of the castle, not taken to the castle.

Compare:

• hacer fotos del castillo = take photos of the castle
• ir al castillo = go to the castle

So del castillo describes the subject of the photos.

Al atardecer means at sunset, toward dusk, or at dusk.

The al is the contraction of a + el:

• a el atardecer → al atardecer

This expression is fixed and very common in Spanish.

Related expressions:

• al amanecer = at dawn
• al anochecer = at nightfall

So quiero hacer fotos del castillo al atardecer means the speaker wants to take the photos at that time of day, when the light is changing.

Because Spanish often leaves out subject pronouns when they are clear from the verb form.

• quiero already means I want

So:

• quiero hacer fotos = I want to take photos

You could say yo quiero hacer fotos, but it usually adds emphasis, contrast, or emotion.

For example:

• Yo quiero hacer fotos, pero ella quiere pintar.
  = I want to take photos, but she wants to paint.

Without special emphasis, leaving out yo is the most natural choice.

Yes, Spanish word order is flexible.

The original sentence:

• Desde el mirador, quiero hacer fotos del castillo al atardecer.

This is natural because it starts with the setting: from the viewpoint.

Other possible versions:

• Quiero hacer fotos del castillo al atardecer desde el mirador.
• Al atardecer, quiero hacer fotos del castillo desde el mirador.

These all make sense, but the emphasis changes slightly:

• Desde el mirador... emphasizes the location first
• Al atardecer... emphasizes the time first
• starting with Quiero... is more neutral

It is not always strictly necessary, but it is very natural here because Desde el mirador is an introductory phrase.

So:

• Desde el mirador, quiero hacer fotos del castillo al atardecer.

This helps the sentence read more clearly.

You may also see it without a comma in less careful writing:

• Desde el mirador quiero hacer fotos del castillo al atardecer.

Both are understandable.

Yes.

• Quiero hacer una foto del castillo al atardecer. = I want to take one photo of the castle at sunset.
• Quiero hacer fotos del castillo al atardecer. = I want to take photos of the castle at sunset.

The plural fotos suggests several pictures, which is very natural in this context.

Spanish uses definite articles more often than English does.

Here, el mirador and el castillo refer to specific, identifiable things:

• the viewpoint
• the castle

If the place is already known from context, Spanish normally uses the article.

So even if English might sometimes sound okay with less explicit marking, Spanish usually prefers:

• el mirador
• el castillo

The original sentence is already natural, but in everyday speech someone in Spain might also say:

• Desde el mirador, quiero hacerle fotos al castillo al atardecer.
• Quiero sacar unas fotos del castillo desde el mirador al atardecer.

A few notes:

• hacerle fotos a algo is also common in speech
• unas fotos sounds very natural for some photos
• sacar fotos is possible too

Still, your original sentence is perfectly good Spanish from Spain.