Después fuimos al castillo, desde donde se veía toda la ciudad.

Después means afterwards / later / then here. It is an adverb used to move the story forward in time.

In this sentence, it does not need de because nothing follows it directly. Compare:

• Después fuimos al castillo. = Afterwards, we went to the castle.
• Después de comer, fuimos al castillo. = After eating, we went to the castle.

So here después simply introduces the next event.

Fuimos is the preterite of ir, and it is used for a completed action in the past: we went.

That fits the sentence because the speaker is narrating a sequence of events:

• first something happened
• afterwards, we went to the castle
• from there, there was a view of the city

If you said íbamos, it would sound more like we were going / we used to go, which does not fit this one-time completed event as well.

In Spanish, a + el contracts to al.

So:

• a el castillo → al castillo

This contraction is basically mandatory, except when El is part of a proper name, such as a El Escorial.

Desde donde literally means from where or from which place.

It refers back to el castillo:

• fuimos al castillo, desde donde...
• we went to the castle, from where...

In natural English, you might translate the whole part as:

• from where the whole city could be seen
• from which you could see the whole city

So desde donde introduces a relative clause connected to the place just mentioned.

Because this is not a question word here. It is a relative adverb, referring back to castillo.

Use dónde with an accent in direct or indirect questions:

• ¿Dónde está el castillo?
• No sé dónde está el castillo.

Use donde without an accent in relative clauses:

• el lugar donde nací
• el castillo desde donde se veía la ciudad

So the lack of accent is correct here.

Here se veía means something like could be seen or was visible.

The se makes the sentence less personal. Instead of saying exactly we could see the whole city, Spanish often uses this kind of structure to focus on what was visible rather than on the people looking.

So:

• desde donde se veía toda la ciudad
• from where the whole city could be seen
• from where you could see the whole city

All of those capture the idea.

The imperfect is used here because the sentence is describing a background situation or state: the castle was a place from which the city was visible.

It is not talking about one single completed act of seeing. It is describing the view.

Compare:

• desde donde se veía toda la ciudad = from where the whole city could be seen / was visible
• desde donde se vio toda la ciudad would suggest a more specific completed event of seeing, which sounds less natural here

So the imperfect fits the descriptive tone.

Grammatically, toda la ciudad is the subject.

That is why the verb is singular:

• se veía toda la ciudad

Even though toda means all / the whole, the noun ciudad is singular, so the verb is singular too.

If the noun were plural, the verb would also be plural:

• se veían las montañas = the mountains could be seen

Toda la ciudad means the whole city.

Here toda agrees with ciudad, which is feminine singular:

• todo el pueblo
• toda la ciudad
• todos los edificios
• todas las casas

So toda la ciudad is the normal way to say the entire city.

The comma marks off extra descriptive information about el castillo.

The main statement is:

• Después fuimos al castillo

Then the sentence adds more information:

• desde donde se veía toda la ciudad

This kind of relative clause is giving additional detail, so the comma is natural.

Yes, absolutely.

• Después fuimos al castillo, desde donde se veía toda la ciudad.
• Después fuimos al castillo. Desde el castillo se veía toda la ciudad.

Both are correct. The version with desde donde is just a more connected, elegant way of combining the two ideas into one sentence.

Yes. Even though Spanish says se veía toda la ciudad, a very natural English translation is we could see the whole city.

Spanish often avoids naming the viewer when it is obvious from context. So the sentence can be understood as:

• from there, the whole city was visible
• from there, you could see the whole city
• from there, we could see the whole city

All are valid depending on how natural you want the English to sound.