Castle Roogna by Piers Anthony

But it seemed he was also that kind of a fool. Maybe it came with growing up. Her talent of sex appeal-
           Dor tapped at the stone. It was distressingly solid. No hollow panels there. He felt for crevices. The interstices between stones were too small for his fingers, and he already knew there were no ledges for climbing. "Got to be in one of those alcoves," he said.
           They checked the alcoves, carefully. There was nothing. The noxious plants grew from stone planters sitting on the rampart; there was no secret entrance through their dirt.
           But the niche of the needle-cactus seemed deeper. In fact it curved into darkness beyond the cactus. A passage!
           Now all he had to do was figure out how to pass one of the deadliest of the medium-sized plants of Xanth. Needle-cactuses tended to shoot first and consider afterward. Even a tangle tree would probably give way to a needier, if they grew side by side. Chester the centaur, a friend of Dor's father, still had puncture scars marring his handsome rump where a needier had chastened him.
           Dor poked his head cautiously around the corner. "I don't suppose you feel like letting a traveler pass?" he inquired without much hope.
           A needle shot directly at his face. He jerked violently back, and it hissed on out to land in the moat There was an irate protest from the triton, who didn't like having his residence littered.
           "The needier says no," Grundy translated gratuitously.
           "I could have guessed." How was he going to pass this hurdle? He couldn't swim under this cactus, or reason with it, or avoid it. There was barely room to squeeze by it, in the confined alcove.
           "Maybe loop it with a rope, and haul it out of the way," Grundy suggested dubiously.
           "We don't have a rope," Dor pointed out. "And nothing to make one with."
           "I know someone whose talent is making ropes from water," Grundy said.
           "So he could pass this menace. We can't. And if we did have rope, we'd get needled the moment we hauled the cactus out into the open."
           "Unless we yanked it right into the moat." Dor chuckled at the thought. Then he got serious. "Could we fashion a shield?"
           "Nothing to fashion it from. Same problem as the rope. This ledge is barren. Now if cacti don't like water at all, maybe we can scoop-"
           "They can live without it, but they like it fine," Dor said. "They get rained on all the time. Just so long as it doesn't flood too much. Splashing won't do any good, unless-" He paused, considering. "If we could send a lot of water flowing through there, flood out the cactus, wash the dirt from its pot, expose the roots-"
           "How?"
           Dor sighed. "No way, without a bucket. We just aren't set up to handle this cactus."
           "Yeah. A firedrake could handle it. Those plants don't like fire: it burns off their needles. Then they can't fight until they grow new ones, and that takes time. But we don't have any fire." He shook a few drops from his body. "Sometimes I wish you had more physical magic, Dor. If you could point your finger and paralyze or stun or burn-"
           "Then the Good Magician would have had other defenses for his castle, that those talents would be useless against. Magic is not enough; you have to use your brain."
           "How can a brain stop a needier from needling?" Grundy demanded. "The thing isn't smart; you can't make a deal with it."
           "The cactus isn't smart," Dor repeated, an idea forming. "So it might not grasp what would be obvious to us."
           "Whatever you're talking about is not obvious to me, either," the golem said.
           "Your talent is translation. Can you talk cactus language too?"
           "Of course. But what has that to do with-"
           "Suppose we told it we were dangerous to it?