Remove the Luck component of searching for secret doors and passages. Specifically, change the rnl() calls that allow a search to fail into rn2() calls, which have no Luck bias.

Rather than occurring entirely randomly, a secret passage can only generate to close off a dead end, where exactly 2 of the 4 adjacent squares, on opposite sides, are corridors, and the other 2 are rock. The dead end criterion is tested by trying to pathfind an alternate route from one of the adjacent corridor spaces to the other; if none can be found, this space must be blocking a dead end, so it is valid.

Scale the probability of secret doors and secret passages in a room-and-corridor level based on its depth, so that level 1 is almost guaranteed not to contain any blocking the way, and the other early levels are also not likely for this to happen.

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.

Use the builtin pathfinding algorithm to trace a path between the upstairs and downstairs of each level in the Dungeons of Doom. Along this path, remove or relocate any traps, and (in early levels, perhaps up until the Oracle) convert any secret doors or passages along the path into regular ones.

A way to search for hidden passages behind a boulder you are pushing.