Why were Achaemenian reliefs placed on external stairways and facades?

In Achaemenian architecture, relief sculptures were primarily placed on external stairways and facades to serve a specific visual function. Unlike earlier traditions where such carvings might appear as interior decoration, Achaemenian reliefs were strategically used to provide visual emphasis to prominent external architectural features.1

By positioning these reliefs on the facades of buildings and the ornamental stairways used to approach them, the architects ensured that the carvings were highlighted by natural light. This placement allowed the bright colors applied to the reliefs to be effectively showcased by the sunshine. These decorative elements, such as the famous relief of tribute bearers on the stairway of the Apadana at Persepolis, served to enhance the grandeur of the imperial structures, reflecting the power and organization of the Achaemenian Empire.123