Montresor walls up Fortunado in the catacombs under the city by sticking him in a crevice and bricking him in.
Montresor is upset because of some perceived injury Fortunado has done him. He tricks him by saying he needs him to look at a rare cask of wine in the catacombs, tunnels under the city. Then he brings him deeper and deeper, and since the man is drunk he is not aware of what is happening until it is too late.
…I busied myself among the pile of bones of which I have before spoken. Throwing them aside, I soon uncovered a quantity of building stone and mortar. With these materials and with the aid of my trowel, I began vigorously to wall up the entrance of the niche.
Fortunado screams, but they are deep below the city and no one can hear him. Montresor has plenty of time to wall him in, and eventually he will suffocate to death and no one will ever know what happened to him or suspect Montresor. Montresor comments that it is important that he commit his act of revenge without getting caught. This is why it is important to commit the perfect crime.