The zendan, a Zoroastrian shrine, also known as "prison of Solomon". An identical monument has been found at Naqš-i Rustam (the Ka'bah-i Zardusht), and the function of the two buildings must be identical. It is often said that they were used to keep the holy fire, but the absence of a chimney at Naqš-i Rustam does not support this interpretation. An alternative is that in these buildings books (e.g., the Avesta) were stored, but many scholars think that in the Achaemenid age, the sacred texts were learned by heart. So we don't know.
http://www.livius.org/a/iran/pasargadae/pasargadae2.html