From "Manual of Muad'Dib" by the Princess Irulan

Hawat certified this place , Jessica thought. There can't be anything too terrible in here .
She pushed the door. It swung inward onto a small room with another oval door opposite. The other door had a wheel handle.
An air lock! Jessica thought. She glanced down, saw a door prop fallen to the floor of the little room. The prop carried Hawat's personal mark. The door was left propped open , she thought. Someone probably knocked the prop down accidentally, not realizing the outer door would close on a palm lock .
She stepped over the lip into the little room.
Why an airlock in a house? she asked herself. And she thought suddenly of exotic creatures sealed off in special climates.
Special climate!
That would make sense on Arrakis where even the driest of off-planet growing things had to be irrigated.
The door behind her began swinging closed. She caught it and propped it open securely with the stick Hawat had left. Again, she faced the wheel-locked inner door, seeing now a faint inscription etched in the metal above the handle. She recognized Galach words, read:
"O, Man! Here is a lovely portion of God's Creation; then, stand before it and learn to love the perfection of Thy Supreme Friend."
Jessica put her weight on the wheel. It turned left and the inner door opened. A gentle draft feathered her cheek, stirred her hair. She felt change in the air, a richer taste. She swung the door wide, looked through into massed greenery with yellow sunlight pouring across it.
A yellow sun? she asked herself. Then: Filter glass!
She stepped over the sill and the door swung closed behind.
"A wet-planet conservatory," she breathed:
Potted plants and low-pruned trees stood all about. She recognized a mimosa, a flowering quince, a sondagi, green-blossomed pleniscenta, green and white striped akarso . . . roses . . .
Even roses!
She bent to breathe the fragrance of a giant pink blossom, straightened to peer around the room.
Rhythmic noise invaded her senses.
She parted a jungle overlapping of leaves, looked through to the center of the room. A low fountain stood there, small with fluted lips. The rhythmic noise was a peeling, spooling arc of water falling thud-a-gallop onto the metal bowl.
Jessica sent herself through the quick sense-clearing regimen, began a methodical inspection of the room's perimeter. It appeared to be about ten meters square. From its placement above the end of the hall and from subtle differences in construction, she guessed it had been added onto the roof of this wing long after the original building's completion.
She stopped at the south limits of the room in front of the wide reach of filter glass, stared around. Every available space in the room was crowded with exotic wet-climate plants. Something rustled in the greenery. She tensed, then glimpsed a simple clock-set servok with pipe and hose arms. An arm lifted, sent out a fine spray of dampness that misted her cheeks. The arm retracted and she looked at what it had watered: a fern tree.
Water everywhere in this room—on a planet where water was the most precious juice of life. Water being wasted so conspicuously that it shocked her to inner stillness.
She glanced out at the filter-yellowed sun. It hung low on a jagged horizon above cliffs that formed part of the immense rock uplifting known as the Shield Wall.
Filter glass , she thought. To turn a white sun into something softer and more familiar. Who could have built such a place? Leto? It would be like him to surprise me with such a gift, but there hasn't been time. And he's been busy with more serious problems .
She recalled the report that many Arrakeen houses were sealed by airlock doors and windows to conserve and reclaim interior moisture. Leto had said it was a deliberate statement of power and wealth for this house to ignore such precautions, its doors and windows being sealed only against the omnipresent dust.
But this room embodied a statement far more significant than the lack of waterseals on outer doors. She estimated that this pleasure room used water enough to support a thousand persons on Arrakis—possibly more.
Jessica moved along the window, continuing to stare into the room. The move brought into view a metallic surface at table height beside the fountain and she glimpsed a white notepad and stylus there partly concealed by an overhanging fan leaf. She crossed to the table, noted Hawat's daysigns on it, studied a message written on the pad:
"TO THE LADY JESSICA—
May this place give you as much pleasure as it has given me. Please permit the room to convey a lesson we learned from the same teachers: the proximity of a desirable thing tempts one to overindulgence. On that path lies danger.
My kindest wishes,
MARGOT LADY FENRING"
Jessica nodded, remembering that Leto had referred to the Emperor's former proxy here as Count Fenring. But the hidden message of the note demanded immediate attention, couched as it was in a way to inform her the writer was another Bene Gesserit. A bitter thought touched Jessica in passing: The Count married his Lady .
Even as this thought flicked through her mind, she was bending to seek out the hidden message. It had to be there. The visible note contained the code phrase every Bene Gesserit not bound by a School Injunction was required to give another Bene Gesserit when conditions demanded it: "On that path lies danger."
Jessica felt the back of the note, rubbed the surface for coded dots. Nothing. The edge of the pad came under her seeking fingers. Nothing. She replaced the pad where she had found it, feeling a sense of urgency.