You quoted from Letter 64:
"[Sam] treats Gollum rather like Ariel to Caliban".
It's an interesting comparison for Tolkien to have used. Ariel and Caliban are
both servants to Prospero. Ariel is graceful and magical, and clearly despises
Caliban. Caliban is clumsy and ugly, and could not even speak before Prospero
taught him. Their names illustrate their natures.
A sample of Caliban's speech:
"When thou camest first,
Thou strokedst me and madest much of me, wouldst give me
Water with berries in't, and teach me how
To name the bigger light, and how the less,
That burn by day and night: and then I loved thee
And show'd thee all the qualities o' the isle,
The fresh springs, brine-pits, barren place and fertile:
Cursed be I that did so!"
There's a hint of Smeagol-as-guide here.
And yet Ariel, for all his sense of superiority, is slave rather than servant.
He is grateful to Prospero for freeing him (he was imprisoned in a tree! Not a
willow, though), but he begs for release.
And Caliban makes a convincing case when he says that the island was stolen
from him.
Nothing is what it seems; no one is what they seem. Mystery and magic are used
in far more subtle ways than in "A Mid-summer Night's Dream".
It's one of the comparatively gentle last plays, so it's not a spoiler to say
there's a happy ending
:-)

Part of my garden
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The Passing of Mistress Rose
Do we find happiness so often that we should turn it off the box when it happens to sit there?
- A Room With a View