The decor of Charles Cameron has been best preserved in the Billiard Room, finished in 1786. Laid out on a square plan, it has a slightly raised ceiling imitating a vault, with a large rosette in the centre, an ornamental border, and bas-reliefs of classical subjects in the corners. The walls, divided into panels, are topped by a cornice with a frieze of delicate design. The absence of gilding, mirrors or painting, and a very light, almost white colouring, add to the severely classical style of the interior.

Voronikhin made no alterations in Cameron's decor when reconstructing the Billiard Room after the fire. The walls are hung with canvases by eighteenth-century Italian painters, including several by Michele Marieschi, the rest being old copies of pictures by Antonio Canale (known as Canaletto): views of Venice, with palaces, churches and other beautiful buildings.

The furniture of the Billiard Room includes a table with a large Wedgwood bas-relief plaque showing Diana and Endymion. The card-tables of inlaid wood are remarkable specimens of the work of Russian mid-eighteenth century craftsmen, while the mahogany furniture with brass trimmings was made by St Petersburg cabinet-makers toward the close of the eighteenth century. A piece of special interest is the organ clavichord, a very rare eighteenth-century musical instrument. Its case is ornamented with a pictorial marquetry of musical instruments, an open music book, etc. On the inside of the lid is another inlaid composition of musical instruments, with a sheet of music, in which the notation may be read; it records the beginning of a Russian song. Ordered by Catherine II for Prince Potiomkin, it was made in 1783 at the Gabrahn workshop in 5t Petersburg.