The rough nature of this plan and the multitude of notes in Soane's hand regarding technical details such as the placement and dimensions of the cistern, suggests that Soane had decided on the plan and room layout and was working out the exact details which would later provide instructions for the workmen (although this version shows a preliminary stage in this process).

The marginal sketch for a lantern was presumably intended for the plaster cast gallery, where two dome skylights are marked out on the plan. The ruins are also labelled, though not designed.