A yellow musk creeper sounds like it would fit. When a victim smells one of its flowers, the creeper puffs pollen in victim's face. The pollen causes the victim to walk toward the center of the plant which then injects a seed into the brain. The victim is now a yellow musk zombie who protects the plant until the seed is ready to sprout.
That could work rather well. I am trying to find things that could be guarding the entrance to a tower. Seeing as the owner is immune to poisons, it seemed that something like this would be the logical path.