" He cannot see me, Bran realized, despairing. He wanted to reach out and touch him, but all that he could do was watch and listen. I am in the tree. I am inside the heart tree, looking out of its red eyes, but the weirwood cannot talk, so I can’t. Eddard Stark resumed his prayer. Bran felt his eyes fill up with tears. But were they his own tears, or the weirwood’s? If I cry, will the tree begin to weep?"
It must’ve broken down long ago… Probably a gardener robot. It protected this place even after the people went away.