Possible Clues:

• Opening in floor
• Stage feature
• Fall location?
• Where one was caught in the act, entering?
• Raised cross section showing stage entrance
• Panel in roof seen in cross-section, from underneath
• Opening in roof or floor
• It provides a quick descent - a drop unexpectedly in height
• Cross-section in elevation for entrance into loft?
• It ensures pantomime characters go down well