Secret door

Unlike the Dungeon Trap, the Secret Door doesn't do any damage. These doors are -as the name says- hidden, and are like this to make the unobservant think that either they've reached a dead end or let them strave to death in a seeming death trap. While some secret doors are just illusionary walls, others are real doors that are hidden by a mechanism and have to be revealed by carefully examinating the walls.

Secret doors always were a staple of dungeons, although by Ultima IV, they also became quite common in houses to shield off secret areas. These doors were at their worst in Ultima V, where the adventurers would have to search every wall, in order to discover secret doors, which didn't show up on the map seen with the spell Peer. After that time, these doors became easier to spot and were less common.

Trivia

 * Despite them being secret doors, in every game starting with Ultima IV, they are marked with an irregular pattern in the 2D-view. This however does not got for the 3D-dungeons in Ultima IV and Ultima V.