Improvements to secret doors:

• When the player discovers a secret door, monsters still don’t know about it unless they see it open.
• Monsters know about some secret doors and passages in their native area, and will use them as if they were normal doors/floor. If the player sees a monster traveling through one (defined as being able to see the actual square of the door), it becomes unhidden. The proposed implementation for this is to have some monster types (the Wizard, angels, etc) have full knowledge of all secret doors everywhere, and all other monsters have a bit that represents whether they know about secret doors, which sometimes gets set when they are generated and always gets set when they see some other monster use a secret door. A cruder implementation is just to convert the secret feature into a normal one when used by a monster.
• There are fewer or no hidden doors between the upstairs and downstairs on a Dungeons of Doom level.
• Less of a Dungeons of Doom level’s loot appears on a direct or near-direct path between staircases, and more of the loot appears in rooms hidden behind secret doors.
• Remove secret passages from the game; they don’t add much of relevance and it’s harder to find ways to make them relevant.