The central place in the ensemble built by Charles Cameron belongs to the pavilion "Cold Bath." The model of the Cold Bath was completed in 1780, and in the spring of the same year, the construction of the pavilion—a small two-story building—began. On its lower floor, there were rooms for water procedures, and on the upper floor, six richly decorated rooms for rest and entertainment, called the "Agate Rooms," were located.

The architectural design of the facades of the Cold Bath, as well as the entire complex, is based on the contrast of the finishing of the floors. The lower, basement floor is separated from the second by a cornice and faced with massive blocks of roughly processed, porous Pudost stone, seemingly eroded by winds and rains, creating the illusion of "antiquity," the authenticity of an "ancient" monument. The second floor, on the contrary, is light and bright; against the gentle yellow tone of the walls, niches painted in terracotta stand out. Along the top of the walls are round molded medallion bas-reliefs with mythological compositions. The longitudinal northeast wall has semicircular windows cut into it, while the end walls have rectangular door-windows placed in arches. In the piers are niches with sculptural figures of various allegorical and mythological characters. The main southwest facade of the Cold Bath faces a terrace on vaults supported by massive brick pillars. The terrace serves as the base for the Hanging Garden and is a connecting link between Cameron’s ensemble structures and the Great Tsarskoye Selo Palace. From the side of the Hanging Garden and Cameron’s Gallery, the upper floor of the Cold Bath is perceived as a completely independent one-story park pavilion. The main facade differs from the other facades by having columns similar to those on Cameron’s Gallery, emphasizing the unity of these structures.

The Cold Bath is a rare example in the history of Russian architecture of a building whose very plan reflects the idea of following ancient models. When developing the project of the Cold Bath, Cameron apparently used the layout of the central part of the Baths of Constantine, destroyed in the early 17th century and known from measurements by A. Palladio, which he reproduced with comments in his treatise "Roman Baths." According to Cameron, the rooms of the Roman baths included the apodyterium, or dressing room; unctuarium—for storing oils; sphaeristerium—a large exercise hall; caldarium—hot bath; laconicum—steam room; tepidarium—warm room with heated water; and frigidarium—cold bath. The Cold Bath included a Bathing Hall with a tin pool in the center, "warm" and "hot" baths, and a "rest room." All rooms on the lower floor have stone vaults and are illuminated by high semicircular "thermal" windows. Throughout, there are simple oak parquet floors and white-painted single-leaf paneled doors.

In the interiors of the second floor, Charles Cameron used colored stone, painting, and molding. The molded work throughout the building was done by Melnikov—a serf of Lvov, a famous architect and poet—and other masters; sculptural work was done by Rashette and Goffert. Other talented master decorators working in St. Petersburg in the 1780s also participated in the interior decoration, among whom the sculptor and decorator J.-B. Charlemagne-Baudet, who acted as a contractor for most of the finishing work, and the painter Rudolf should be especially noted.

For the floors on the upper floor, parquet floors made by master Meyer according to Felten’s designs for the Lansky house, built in the same years in St. Petersburg (due to the death of the empress’s favorite, they remained unused), were used. Despite damage during the war years, the original interior decoration of the pavilion has been preserved. Currently, the rooms on the first floor of the Cold Bath are used for temporary exhibitions. From the entrance located in the middle of the building under the vaults of the Hanging Garden, a corridor begins, along the sides of which various service rooms were located: rooms for servants, storerooms for oils and various bath accessories, rooms where furnaces and boilers for heating water were arranged.

Today, we enter the rooms of the first floor through a small oval vestibule with niches similar to those on the facade. Immediately behind it was the "hot bath," arranged as a traditional Russian steam room, small and previously entirely lined with wood. Cameron had no idea about the arrangement of the Russian bath and turned to the Construction Office for help in its creation. The work was entrusted to architect I. V. Neelov, who presented the estimate to the clients in October 1785. According to his project, a stove was placed in the southern corner of the room, on the grating vault of which 250 cannonballs were stacked; they were watered to create hot steam. In the opposite corner was a bench made of linden boards, benches, and water tanks. Heated water came from a boiler located behind the wall. The empress descended to the Cold Bath by stairs from the bel étage, passing from her rooms through the Hanging Garden.

Two corner cabinets have identical decoration. Only the elegant Corinthian columns of colored and white marble, conceived by Cameron, were executed. These columns support the springing of the vaults, on the pendentives of which round molded bas-reliefs are placed. Two of them are devoted to the myth of Cupid and Psyche, two others depict Aphrodite and Adonis and a scene from the story of Selene and Endymion. The cabinet closest to the staircase was called the "rest room" and was almost entirely occupied by an "antique couch." Here, a white marble fireplace with female figures on the pilasters and carved ornament on the frieze, the work of Charlemagne-Baudet, is excellently preserved. The second corner cabinet, located between the warm and steam baths, was intended for massage procedures. Adjacent to the "massage room" was the bathroom or "warm bath"—a spacious and bright room where there was previously a bath for bathing in warm water. The walls of the Bathroom were plastered and painted; their only decoration was profiled frames dividing the wall surfaces into panels, and molded decorative vases in high relief above the door pediments, two of which are false and serve a decorative function. In the 19th century, a light gray marble fireplace was installed in the Bathroom.

The most spacious and bright room on the first floor is the Bathing Hall, covered with a double cross vault. According to Cameron’s original project, the Bathing Hall with the pool was supposed to look especially festive, dazzling with gilded molded decorations on the ceilings and also rich wall decoration made of artificial marble, painted with ornaments. It was planned to install a luxurious canopy over the pool on faience columns with gilded bronze eagles and to pave the floor with multicolored marble. However, the empress rejected this project; Cameron managed to defend only part of his original idea and decorate the walls of the hall with relief compositions on mythological themes related to the water element. Immediately under the vaults are large semicircular panels with multi-figure compositions: on the southeast wall—"Acis and Galatea," on the southwest—"Pan and Syrinx" and "Centaur Nessus and Deianira." The bas-relief on the northwest wall depicts Amphitrite surrounded by nereids, tritons, dolphins, and cupids. Below runs a continuous sculptural frieze composed of rectangular panels alternating with round medallions, depicting such subjects as "Venus’s Toilet," "Galatea and Neptune," "Bathing Naiads," "Triumph of Amphitrite," as well as allegories of rivers and lakes, images of bathers, and the muse Euterpe. The sculptural compositions were executed under Rashette’s supervision. Under the parquet floor of the hall is hidden a brick frame of a round pool for bathing in cool water with a volume of 13 cubic meters, surrounded by a wooden balustrade. Steps led down into a tin bath made by master Albrecht. A white marble fireplace decorated with gilded bronze was installed in the hall.

During the Great Patriotic War, a stable was arranged in the Bathing Hall; the pool was broken, many doors destroyed, and only fragments of the floors remained. Restoration work was carried out in 1949 and 1990.

The two floors of the "Cold Bath" pavilion are connected by a spiral staircase seemingly hanging in the air. Its spiral flight is fitted into the oval volume of the room without the usual supporting walls. The ends of the steps are inserted into a special channel carved into the wall and wedged with stone; the edges of the steps are connected by rounding and recesses, so the steps also rest on each other. Cost-cutting on the artistic decoration of the rooms on the first floor of the Cold Bath and revision of design solutions also affected the staircase. To Cameron’s question: "What should the steps be made of, Russian marble or wild stone?" came the answer from Empress Catherine II: "Of wild stone, but only calm." Fulfilling the client’s wishes, the architect made the steps from gray fine-grained granite. The staircase railing received a simple design in the form of vertical rods and smooth gilded rosettes; its polished handrail is made of red Virginia wood. The floors of the stair landings are laid with white and gray marble with a wide border of Putilov slabs. The walls of the stairwell are pierced by two doors and a pair of windows, between which are semicircular niches with marble sculptures. Above the niches are round molded medallions with mythological compositions by Rashette. The glass filling of the window-door leading to the Hanging Garden, curved to the shape of the wall, was restored in 2012.

The white rosettes of the dome coffers above the staircase create the impression of a lace pattern. The base of the dome is surrounded by a molded cornice with a relief of lotus leaves, and below it stretches a wide frieze of elegant arabesques with figures of griffins and vases between them. The silhouette of the staircase and its entire space, created by Charles Cameron, constitute one of the most perfect and exquisite creations of the Scottish architect, unique in the history of Russian architecture, with which his work turned out to be connected.

Sources:

https://ru.wikipedia.org/wiki/Cold_Bath_with_Agate_Rooms

https://www.tzar.ru/objects/ekaterininskypark/cameron/coldbathhouse