If you are still looking for something nasty but reasonable?

A password-sprung trap. Say the right word, and there's no problem. Do any of them speak elvish? Doesn't even have to be hard, just "Excuse me" in elvish. You know, politeness?

Fail, and you find out that it is, in fact, a *living maze*. All of those hedge walls become limbs that activate and try to squish you and hold you.

Not necessarily kill. You said this was a front-door guard against intruders, right? Mess up, and wind up captured, held, and then the palace guards come out and ... now you have to deal with politics and such.

Of course, that's when the smart wizard uses their saved fireball. And that will make the guards that come out really made at you.