There's multiple ways of answering the riddle, but if you take into account the "don't overthink it", the simplest answer should be the correct one.
However,
@mommunism the trap is working perfectly if the enemy are wasting both time and monsters on it. I recall one story which bade adventurers to answer an obscure riddle, and the answer to the riddle is "the trap is sound activated, fool, if you shout your answer out the trap activates and kills you". People expect to be rewarded for a correct answer for a riddle. Thus if you lure them into trying to solve your puzzle, whether there is actually a solution or not, you're winning.
An adventuring party might see a trap door and bypass it with a ladder, but an adventuring party won't bother with the ladder if there's a riddle, they'll keep trying to solve the riddle until they start complaining to the GM (unless they're smart and realise it's just another trap to get folks killed)