How was the Parthenon changed when it became a Roman Catholic cathedral?

When the Parthenon was converted into a Roman Catholic cathedral during the period when Athens was held by the Crusaders (beginning in 1204), the most notable addition was the construction of a bell tower.1

Prior to this, during its earlier transformation into a Byzantine church, the building underwent structural alterations in its inner portion, and several of its original sculptures were removed, including the colossal statue of Athena created by Phidias. These changes reflect the building's long history of repurposing, which saw it transition from a pagan temple to a Christian place of worship, and later to a mosque under Ottoman rule.23