Montresor describes his chaining of Fortunato in fairly precise detail. There is a very narrow niche in the granite wall of the catacombs.
Within the wall thus exposed by the displacing of the bones, we perceived a still interior crypt or recess, in depth about four feet, in width three, in height six or seven. . . . In its surface were two iron staples, distant from each other about two feet, horizontally. From one of these depended a short chain, from the other a padlock. Throwing the links about his waist, it was but the work of a few seconds to secure it.
Montresor did not install these chains. They had been there for centuries and used by feudal lords to dispose of disobedient vassals, who would simply disappear and never be heard from again. Montresor must have gotten the idea of killing Fortunato this way while exploring the catacombs. Concealing the crypt or recess with a stone wall was his own innovation. Since it is only three feet wide and six or seven feet high, it will be easy to build the wall.
One chain is only two feet long. The other is only a stub of a chain with a padlock. Montresor only has to draw the longer chain across Fortunato's body and secure it with the padlock. The recess is four feet deep, so the victim will be unable to interfere with the construction of the wall or knock it down after it is finished but still damp. His arms would only be about three feet long, so the wall would be a full foot beyond his reach.
He is pinned tightly to the wall. He has no chance of slipping out from under the chains or of climbing over them. Montresor has specified that Fortunato is wearing a tight-fitting jester's costume. He has no flexibility in such an outfit, and he probably doesn't even have any pockets. He has no weapon and nothing he could use as a tool to try to file a chain or pick the padlock. The padlock is old-fashioned. It is locked with a key rather than snapping shut like our modern padlocks.
Withdrawing the key I stepped back from the recess.
Fortunato would find it impossible to pick such a padlock even if he had some sort of tool. His death will be agonizing because he will have to stand in one upright position until he dies of starvation. Montresor repeatedly specifies that there is a lot of dripping water, so Fortunato will be able to lick drops off his hands and off the rock wall. It takes much longer for a man to die of starvation than of thirst.
Montresor has an easy time chaining Fortunato because of the simplicity. He also has an easy time building the stone wall because it only needs to be three feet wide. He is not using bricks but stones. The stones are heavy and may be almost a full foot long, so three or four stones would make up a whole tier. He has already prepared the mortar (because Poe wants this part of the story to end quickly); and the mortar stays damp because of all the dripping water. Montresor keeps the trowel on his person because he doesn't want it to rust. At the same time, he doesn't want to have to go someplace to fetch it when he brings Fortunato home. He uses the remaining mortar he has mixed in the trough to "plaster over" the entire stone wall and make it look like part of the catacomb wall itself. Then he covers the fake wall with what he calls a "rampart" of human bones. He is writing or telling this story fifty years after the event, and in all that time no one has discovered what is left of Fortunato.