On the shore of the Mirror Pond stands the pavilion "Upper Bath," or as it was called in the 18th century, "Their Highnesses' Soaphouse," built between 1777 and 1779 by the Neelov architects. The Upper Bath is executed in the style of early classicism. The sparsely decorated facade creates an impression of refined simplicity thanks to the proportional relationship between the main volume and the three-sided risalit facing the pond. The walls of the risalit are pierced at the bottom by semicircular windows and a wide doorway, and at the top by round windows. The building is crowned with a parapet with a balustrade. The facade completes the perspective of the pond and stands out clearly among the greenery of the trees, while the walls are reflected on the water surface of the pond. The ceiling and walls of the rest hall were painted by Alexey Belsky with copies of the frescoes from Nero's Golden House in Rome. By the end of Catherine II's reign, the bath became known as Their Highnesses' Soaphouse and was used for its intended purpose until the mid-19th century. The "Soaphouse of Their Highnesses" had six rooms: a bath, steam room, vestibule, dressing room, central octagonal rest hall, and a boiler room (stoker's room).

Until the mid-19th century, the Upper Bath retained its original purpose and consisted of six rooms: vestibule, dressing room, bath, steam room, stoker's room (boiler room), and the central octagonal rest hall. The ceiling and walls of the central hall were painted by Belsky based on engravings from drawings by the artist Smuglevich and architect Brenna, who had executed them from the frescoes of Nero's Golden House in Rome.

In 1777, during the reconstruction of the church wing of the Catherine Palace, the imperial family's bath located there was demolished. Therefore, construction began on a new bath, which was called the New Bath. The construction was carried out by father and son Neelov. On the shore of the Mirror Ponds, they erected an elegant pavilion, almost devoid of decorative ornaments. Its beauty is achieved through the successful proportions of the main building body and the three-sided projection (risalit) facing the pond. The pavilion was built in the style of early classicism and is located on the shore of the Mirror Pond. A particularly beautiful view of the pavilion opens from the central alley of the Regular Park.

An interesting historical incident is connected with the Upper Bath. In the summer of 1780, it was turned into a mock tavern for several days. The Roman Emperor Joseph II (Count Falkenstein) was coming to visit Catherine II. He had the habit of staying only in taverns and nowhere else. In Tsarskoye Selo, there was one tavern run by a Frenchman, but apparently, it did not inspire the Empress's confidence, so she ordered the bath to be converted into a tavern. The room with the bath was locked, necessary furniture was brought in, a signboard was hung, and a tavern keeper was appointed. The guest stayed in this fake inn for five days and was satisfied, not noticing the "trick."

After a major renovation carried out in 1911, by order of Emperor Nicholas II, the pavilion was adapted as the Museum of the 200th Anniversary of Tsarskoye Selo. Busts, engravings, old views of Tsarskoye Selo stored in the Tsarskoye Selo palaces, some interesting archival documents of the palace administration, as well as gifts from deputations on the day of the 200th anniversary of Tsarskoye Selo, were transferred here.

During the war, the pavilion was heavily damaged, but the walls survived, and the main hall retained all its paintings. Restoration was carried out in 1952–53 according to the project of architect Novopol'tsev. Artist-restorers under the guidance of Treskin restored the decorative painting, of which only fragments remained after the fire. The Upper Bath, destroyed during the war, was restored in 1952–1953. The ceiling and two lunettes of the central hall, as before, depict scenes from the ancient myth of Phaethon, son of the Sun god. The polychrome wall painting, recreated at the same time, uses motifs of garlands, flowers, and fruits, giving the interior a festive and elegant look. After the Great Patriotic War, the pavilion was used as a reading room; people gathered inside and around the building with books and magazines, and chess players sat at the tables. In 2005–2006, the pavilion underwent a major restoration. Currently, temporary thematic exhibitions are held here.

Sources:

https://pushkin.spb.ru/encycl/parks/verhnyaya-vanna.-myilnya-ih-vyisochestv.html

https://www.tzar.ru/objects/ekaterininskypark/regular/upperbathroom