Actors perceiving other actors and the dungeon level.

Visibility works according to KISS. Everything that player sees is real. There are no unmarked hidden tiles and only solid tiles can be marked, so there are no invisible walls and to pass through an illusory wall, you have to use a turn bumping into it first. Only tiles marked with Suspect can turn out to be another tile. (So, if all tiles are marked with Suspect, the player knows nothing for sure, but this should be avoided, because searching becomes too time-consuming.) Each actor sees adjacent tiles, even when blind, so adjacent tiles are known, so the actor can decide accurately whether to pass thorugh or alter, etc.

Items are always real and visible. Actors are real, but can be invisible. Invisible actors in walls can't be hit, but are hinted at when altering the tile, so the player can flee or block. Invisible actors in open space can be hit.