Du haut de la falaise, on voit la mer.

Du haut de is a set expression meaning from the top of, from high up on, or from the top part of something.

So in Du haut de la falaise, the idea is:

• de = from
• le haut = the top / the upper part
• du = de + le

Literally, it is something like from the top of the cliff.

These two expressions are close, but they are not used in exactly the same way.

• du haut de la falaise emphasizes the viewpoint: from the top of the cliff
• en haut de la falaise emphasizes location: at the top of the cliff

So:

• Du haut de la falaise, on voit la mer. = From the top of the cliff, you can see the sea.
• On est en haut de la falaise. = We are at the top of the cliff.

In your sentence, French prefers du haut de because the sentence is about what can be seen from that position.

Here, on is an indefinite subject. It often means:

• one
• you
• we
• people
• someone, depending on context

In this sentence, on voit la mer means something like:

• you can see the sea
• one can see the sea
• the sea is visible

English usually does not translate this on as literal one in everyday speech. A more natural English equivalent is often you or a passive-style idea like the sea can be seen.

Because on always takes third-person singular verb forms in standard French.

So:

• on voit
• on est
• on va

Even when on refers to we in meaning, the verb still stays singular:

• On voit la mer = We can see the sea / You can see the sea

This is one of the most important things to remember about on.

The comma separates the introductory phrase from the main clause.

• Du haut de la falaise = introductory setting / viewpoint
• on voit la mer = main statement

English does the same kind of thing:

• From the top of the cliff, you can see the sea.

The comma is not always absolutely mandatory in every short sentence, but it is very natural and helps readability here.

Yes, that is possible.

Both are natural, but the emphasis is a little different:

• Du haut de la falaise, on voit la mer.
  Starts with the viewpoint or setting: From the top of the cliff...
• On voit la mer du haut de la falaise.
  Starts with the main action: You can see the sea...

French often moves a phrase to the front to highlight it or set the scene first.

Because la mer is the direct object of the verb voir.

• voir quelque chose = to see something
• So: voir la mer = to see the sea

You do not use de after voir in this meaning.

Compare:

• Je vois la montagne. = I see the mountain.
• On voit la mer. = You can see the sea.

Yes. Falaise is a feminine noun, so it takes la in the singular:

• la falaise = the cliff
• une falaise = a cliff

That is why the phrase is de la falaise:

• du haut de la falaise

A simple pronunciation guide is:

dy o də la fa-lèz, on vwa la mèrr

A few key points:

• du sounds roughly like dy but with a French u
• haut is pronounced like o
• falaise sounds like fa-lèz
• voit sounds like vwa
• mer sounds like mèr

There is also a natural flow between words:

• Du haut de la falaise is said smoothly as one group
• on voit la mer is another group

No. Here, du is the contraction of de + le.

French combines them:

• de + le = du

So:

• du haut = de le haut = from the top / from the upper part

This is different from partitive du in something like:

• Je mange du pain. = I am eating some bread.

Same form, but different grammar.

No, it is completely normal and natural French.

A few notes on style:

• on makes it sound very natural and common
• du haut de is standard and idiomatic
• the whole sentence sounds like ordinary descriptive French

So this is a good sentence to learn as a model for describing what can be seen from a particular place.