What is ironic about the fort that the Indians built?

A fort is supposed to be a place of safety, a haven to protect the people inside its walls and battlements from the ravages of war. Yet in "The Devil and Tom Walker" it's anything but. As well as not having done any favors for the Indians themselves in days gone by, it's now become a place where greedy men sell their souls to the devil. When foolish souls venture out to this neck of the woods in search of buried pirate treasure, they are placing themselves in a vulnerable situation, virtually crying out to be tempted by the Devil himself. Just as the fort couldn't protect the Indians, now it cannot afford protection to the mortal souls of those such as Tom Walker.