Secret door

Unlike a potentially deadly dungeon trap, a secret door doesn't do any damage.

Description
These are merely doors that are hidden from view, leading some unobservant adventurers to believe they have reached a dead end, and others to starve to death in what appears to be a death trap with no exits. While some secret doors are just illusionary walls, others are real doors that are hidden by a mechanism, requiring a careful examination of the area to reveal their existence.

Secret doors have been a staple of dungeons since the beginning of the Age of Darkness, although by Ultima IV, they also became quite common in houses to shield off secret areas. This practice reached its peak in Ultima V, where it was necessary to search every wall in order to discover any secret doors that might be present, since the Peer spell was unable to reveal them. After that time, these doors became easier to spot, and were less common.

Trivia

 * Despite them being secret, in every game starting with Ultima IV, these doors are marked with an irregular pattern in the 2D-view. This does not hold true for the 3D-dungeons in Ultima I, Ultima IV and Ultima V, which is what makes secret doors in them so frustrating.