A corpse door is a bricked up opening in a home that could be reopened periodically to remove coffins, usually feet first, and then closed again, to prevent the dead from accessing the home. It was firmly believed that the dead could only return the way they had come, and by carrying the body out feet first, the living further protected themselves from the dead by denying them a clear view of the path taken to burial.
In Denmark, the houses were built with the corpse door made in a keyhole-shape which was also bricked up when not in use. Interestingly, many rune-stones and picture-stones are shaped like large keyholes, suggesting that the door, and the gravestone themselves were seen as the physical embodiments of the doorway between life and death.
Corpse doors were common in superstitious homes all over the world in the early part of the last century, and are still in use in some of your more draugr infested areas.