The zendan ("prison") of Pasargadae is often called a Zoroastrian shrine, and may indeed have played a role in the ancient Iranian religion. The problem is that we do not know which role.
An identical monument has been found at Naqš-i Rustam (the Ka'bah-i Zardusht), and it is a reasonable assumption that the function of the two buildings is identical. It has been assumed 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 modern scholars think that in the Achaemenid age, the sacred texts were learned by heart. A recent theory is that these buildings played a role in the cult for dead kings.
Like so many buildings from the pre-Islamic period, the zendan was rebaptized with a name that connected it to king Solomon: this was the "prison of Solomon".