I am a huge fan of including puzzles in dungeon designs. I feel it provides a change of pace from combat encounters and presents the players with a different form of challenge. However, some designers tend to avoid adding puzzles into an adventure for the simple fact that puzzles don’t make a lot of sense. Why would the in-character designer of a dungeon include puzzles? There are many good reasons for this that do not break the “reality” of a dungeon.
One thing to remember is that puzzles are impediments that the designer of the complex put in place to allow people to overcome the impediment. Unlike traps and monsters that are there to kill and prevent a person from proceeding forward, puzzles are only designed as temporary stops to the progress of whoever is trying to get past the puzzle.
The designer is willing to allow people to pass…as long as he feels they are of sufficient intelligence. This sort of test is often used when the designer wants whatever lies at the end of the puzzle to be found but only by the “right” people. This is often done when the designer expects to die soon but still wants someone to eventually find his treasures.
A variation on this is when “invaders” are being tested in another category other than intelligence. For example, if the designer is a priest hiding a relic, the puzzles will be centric to the faith of that priest. In this case the designer would be testing piety and not intelligence.
-Not a Puzzle
The designer wants a barrier that is easy to bypass if the person knows the answer already. The principal here is that the people in the complex are supposed to be there and already have the answer before they get to the puzzle. This sort of puzzle is used when there are other occupants, such as guards or family, in the complex. Of course that asks the question…why make it a puzzle and not simply a matter of giving the password? See the next point.
The barrier requires the right password but the designer is forgetful. In this case the puzzle is there to jog the memory of what the password is. This also works if the designer is anticipating other people, who they have already given the password to, also to have potentially forgotten the password. In this case the puzzle is a redundant backup.
The designer feels they are intellectually superior to everyone else and wants to show it. The puzzles are a form of taunting. These sorts of puzzles almost always have a painful response for answering a puzzle incorrectly.
-Just Plain Crazy
Sometimes, the designer is simply crazy and gets pleasure from these puzzles. Instead of simply using traps, they use puzzles because in the back of their deranged mind they still want people to move past the barrier.