If you want to understand the film better, I highly recommend those books (in that order) :

- Sergio Leone, Something To Do With Death by Christopher Frayling
- Conversations With S.L. by Noel Simsolo
- Sergio Leone by Oreste De Fornari

Leone explains what he intended to achieve with this movie.
To put it in a nutshell, he wanted to shoot "a fairytale for adults" in which the main character is TIME. The final image of De Niro smoking opium and laughing leads to believe that the whole story existed only in his imagination but... who knows ??? Leone voluntarily created this ambiguity and that's why the movie is so great : the viewers can decide for themselves, they/we can interpret the images the way we want, the story is open, and thus they/we become storywriters in our turn.

BTW, did I mention I was crazy about this film ?!

"Come heavy or not at all." Uncle Junior to Tony S.
"Nenti dire ca nenti si capi" come disse quello. (Say nthg when U know nthg.)
"Chi non ci vuole stare, se ne vada." (If U don't like it here, go somewhere else.)