Well, he didn't make it himself. He outsourced much of the structural fantasizing to his gangly, scrambling daughter and the entirety of the handiwork to Mr. Basilberry Glumley, who was only a little miffed about Ambrosius not helping with the construction of his daughter's dream playground. He knew how much his dear friend didn't like to get his hands dirty. At least, not without a sturdy pair of gloves and there were no gloves that fit the Lord's hands at the moment; they were being repaired by Mrs. Meglantine Glumley née Vice who was, as a rule, miffed most of the time by something or other.
They (meaning Basilberry and the household staff, minus Meglantine who refused to lay a finger on the project out of principle for she presciently considered the whole affair just asking for tragedy) built the princeling's haven around the spine of the seeping willowbane, the dismally gorgeous and ominously elegant growth she was named for and which was encircled by the manor, a panopticon so the girl could be kept an eye or four or eight on. She was prone to somnambulistic flights of fancy and the entire house was outfitted with permanently manned mechanical bits and bobs to stop her from throwing herself down flights of stairs in case she decided she could fly that night, or day, occasionally, though previous attempts had not been successful. The sentinel servants operated on a rotating schedule to ensure the Lord and Lady's often-ill offspring didn't offhandedly off herself. She was remarkably persistent, which gave everyone around her severe agita, especially poor Meglantine, for she was indebted to Dr. Spriggon Tallory, but excepting Lady Ava, for she was much more concerned with scheming the next ball they would host. Diligently sewing her multi-stage-transformative showstopper gowns took weeks and nearly all of her attention. She often would not eat during these stages of creation, and so it also fell to Meglantine to coax her out with broth and oats and stewed greens when she could. There were multitudes of household servants but (un)fortunately for Meglantine, she was the only one Ava trusted to not poison her.
Princeling Willowbane was seven when they finished the treehouse and it had taken a whole year to finally complete, and even then they all knew, wearily, that it would never be complete, only abandoned until Willow had another idea. And oh. There were so many ideas and only so many hours in the day. It seemed to the household that she had many more hours than they did somehow. At heart they all were craftspeople with secret desires of what they would do with their lives if only they had the chance. It seemed only Willow understood that this was the chance and they already had it. But she was seven, so that revelation, even she could articulate it, had a spun-sugar's chance in flame of landing. Much in the same way she would never land on the ground as long as the mechanisms of the manor operated as they intended to, and the Glumleys were excellent makers; clockmakers, toymakers, puppeteers, master and mistress of the house, of the machine built to keep them all safe. From what? Oh, too many things to list for now, but there is indeed a list.
The seeping willowbane was curious in that it was very far removed from where it ought to be, and yet persisted. The hill the manor was built on stretched high into the sky, a small mountain above the sticky-sweet jeweled swampwoods, and so offered a cool, airy haven from the densely-steaming, acidic depths. Yet despite the seemingly ill-fitting climate, the tree in the middle of the courtyard flourished wonderfully. This was a problem because any contact with the wretched growth was sure to bring blistering pustules and a vibrant, multihued rash, within seconds for those who had never brushed up against it before. That the princeling wanted her treehouse built in and around the swaying, leaking thing was terrible. Nevertheless they had to oblige, for it was her will, and she was immune to it anyhow, somehow.
They had figured out she was unaffected by the willowbane in an obtusely morbid fashion. Her mother, post-partum and terribly depressed, had contemplated dropping the newborn in the plant. Swaying like the willowbane itself against the horizon, holding out the child, she decided against it, but her nerves were already frazzled and she was startled by Ambrosius's cry of horror from across the yard. And so she, very much by accident, to be clear, did drop Willowbane into the dripping thicket of thorny, vinelike branches, which twisted in on themselves much like a bird's nest. She had to be dragged away shrieking by Basilberry, while Meglantine was left the gruesome task of pulling out the surely-swollen-to-death child, only to find that she was perfectly unblemished if not a little scratched up. So, the whole affair was treated as a divine blessing, for Dr. Tallory confirmed there were cases of people have less severe reactions and it was really quite extraordinary that the child remained unmarked. Though, to say the lady of the house needed a severe intervention after the excitement had settled down, was an understatement, and a story for later.
Oh, little bird, they sighed collectively, as Willowbane flitted in and around the tree during her youth, and even moreso during and after the construction of the marvelous treehouse. They were understanding of her affinity for the tree, but resented having to work around it. Dr. Tallory was on call constantly for tinctures and ointments and eventually set up a small office in the manor for how frequently he was called upon, not the least to keep an eye on Meglantine, which they both resented as well. The household staff had to clean her clothes constantly, and her flouncing frocks often billowed in the breeze on the hill like clouds. But the princeling loved the tree and for all intents and purposes the tree had to have loved her too for how ripe the fruit was every year. Only she could eat it raw, sink her small, sharp teeth into tart, winedark flesh and stain rivulets of juice onto her dresses, but oh, did it make a delicious jam when stewed with sapsugar and ground rosebark.