A flat character is one that remains the same throughout a work of literature. In other words, this type of character does not experience any changes in his/her behavior, views, or goals. On the other hand, a round character develops as the story unravels, and undergoes changes in feelings, purpose, and/or approach to life.
Flat characters are usually a literary device that allows the writer to highlight the changes in round characters. Thus, flat characters are usually secondary or supporting figures.
However, in The Cask of Amontillado you are dealing with only two characters. A careful reading of the story will show you that, even if at face value one is a victim and the other an executioner, Montresor and Fortunato are alike in a number of ways, starting by their names, which refer to "my treasure" and "fortunate/fortune" respectively.
Montresor tells the story of his crime fifty years later. If the text shows you that he regrets what he has done, you could say that, to some extent, he is a round character. On the other hand, if you can read that Fortunato finally stops fighting his fate and defeats Montresor by refraining from begging for his life, you could say the same about him.
But you need to be very careful and remember that Poe included the idea of "the double" in many of his stories. If these two characters are a variation of the double, you need to make the same decision about both.