Although the "you" is never identified, we can assume that the narrator, Montresor, is revealing his ghastly secret many years in the future. Montresor seems to be telling the story to either a confidante,
You, who so well know the nature of my soul...
or a relative. We do know from the final paragraph that Montresor's secret has never been discovered.
For the half of a century no mortal has disturbed them. In pace requiescat!
So, Montresor not only successfully implemented the perfect crime, but he was able to hold his tongue about his misdeed for 50 years. Montresor is an old man when he finally tells his tale, and it could be as a sort of confessional before he dies himself. Though there is no evidence of it in the story, I have always felt as if Montresor was revealing his dreadful secret to another younger family member, passing down his act of revenge to the next generation.