Here's my interpretation of the riddle.
Like an ancient monument
Said to be hard to get
But you can drink of it
as often as you like
A description of the answer, perhaps. I think the last two lines refer to a place where one can find random encounters.
But without enough cups
At least at the time
Once you get enough
You can't go back for it
The answer is something that is miss-able?
So instead take the last
Of the empty cups
Give them to one and
Go straight to the top
A description of a method to get the miss-able thing?
My guess is the Trine E.Skill and the battle against Godo at the Pagoda.
