Nos sentamos en un banco desde donde se ve el castillo.

Because the verb here is sentarse, which means to sit down.

• sentarse = to sit down
• sentar = to seat someone, or sometimes to suit someone

So:

• Nos sentamos = we sat down
• Sentamos by itself would usually suggest we seat something/someone, which is not the intended meaning here.

The nos is the reflexive/pronominal part that belongs to sentarse.

It is the preterite, first person plural, from sentarse.

So nos sentamos means we sat down as a completed action in the past.

Why preterite? Because the sentence describes a specific event:

• first, we sat down on a bench
• then, from that bench, the castle was visible

If you used the imperfect instead:

• nos sentábamos

that would usually mean something more like:

• we used to sit down
• we would sit
• or it could describe an ongoing background action in the past

Because Spanish normally says sentarse en for chairs, benches, sofas, etc.

So:

• sentarse en un banco = to sit on a bench

This is one of those places where Spanish and English use different prepositions.

Using sobre here would sound too literal, like on top of the bench as a surface, rather than the normal way you say you are sitting on it.

Yes, banco can mean several things depending on context, including:

• bank
• bench
• pew
• workbench

In this sentence, because of Nos sentamos en un banco, it clearly means bench.

Context tells you which meaning is intended.

Desde donde means from where or from which.

Here it refers back to un banco, which is a place. So the structure means:

• a bench from where the castle can be seen
• more natural English: a bench from which you can see the castle

It is a very natural way in Spanish to connect a place with what can be seen, done, or accessed from that place.

Because donde here is a relative adverb, not a question word.

Use dónde with an accent in:

• direct questions: ¿Dónde está...?
• indirect questions: No sé dónde está.

Use donde without an accent in relative clauses:

• el lugar donde nací
• un banco desde donde se ve el castillo

So in this sentence, donde is correct because it means where/from which, not where?

Because se ve el castillo focuses on the fact that the castle is visible from that place, not on who is seeing it.

It is a very common Spanish structure. In natural English, it often comes out as:

• you can see the castle
• the castle can be seen
• the castle is visible

Compare:

• vemos el castillo = we see the castle
• se ve el castillo = the castle can be seen / you can see the castle

So se ve sounds more general and descriptive.

It is often explained as a passive-like or passive se construction, though in practice learners can often think of it simply as a natural Spanish way to say can be seen or is visible.

Here the important thing is:

• el castillo is singular
• so the verb is singular: se ve

If it were plural, you would usually get:

• se ven los castillos = the castles can be seen

So yes, it behaves a lot like a passive structure.

Yes, you could.

• desde donde se ve el castillo
• desde el que se ve el castillo

Both are grammatically possible.

But when the antecedent is a place, donde often sounds more natural and idiomatic. So desde donde is a very good choice here.

You can think of it like this:

• desde donde = more place-based, more natural
• desde el que = more literal from which

Because Spanish usually uses the definite article more often than English with specific nouns.

Here el castillo refers to a particular castle that is known or identifiable in the situation.

So Spanish naturally says:

• se ve el castillo

Where English might say:

• you can see the castle

In some contexts English can drop articles more easily than Spanish, but Spanish generally keeps them with specific countable nouns like this.

Literally, yes, that is the historical idea behind the reflexive form. But in normal modern Spanish, you should usually understand sentarse simply as the standard verb to sit down.

So although nos sentamos could be analysed word-for-word as we sat ourselves, the natural meaning is just:

• we sat down

This is common with many pronominal verbs in Spanish: the reflexive pronoun is part of the verb’s normal form, and you should not always translate it literally.