Why is the ending of The Cask of Amontillado a situational irony?

The ending of "The Cask of Amontillado" by Edgar Allan Poe is considered situational irony because the reader discovers that Montresor has indeed carried out his revenge plan effectively against Fortunato in a way that is unexpected and shocking. The true nature of Montresor's intentions is only revealed at the very end of the story, creating a twist that highlights the dark and ironic outcome of the tale.