Today Mount Sion designates the part of the western hill projecting out beyond the south wall of the Old City in the area of Sion Gate; it is bordered on the west and south by the Hinnom Valley and on the east by the Tyropoeon Valley. In the Old Testament period Sion was the eastern hill, David ‘captured the stronghold of Sion, and it is now known as the City of David’(2 Sam.5:7). The name changed in the C4 AD, presumably on the basis of such passages as Mic. 3:12: ‘Sion shall be ploughed as a field, Jerusalem shall become a heap of ruins, and the Mountain of the Lord [i.e. the Temple Mount] a wooded hill’. The prophet intended to say the same thing in three different was but Christians, such as the Bordeaux pilgrim (333) who quotes the text, took it as a description of the two hills on which Jerusalem is built; if the eastern hill was the Temple Mount, Sion had to be the western hill.
The C6 AD Madapa map gives a surprisingly detailed picture of Mount Sion as it was in the Byzantine period. From the city gate the wall turns past the Tower of David to the south, enclosing the church of Sion and the church of Siloam. The colonnaded street, the CARDO MAXIMUS passes in front of the church of New St Mary, the Nea, and terminates at a gate in the old wall of Aelia Capitolina.
Source: The Holy Land by Jerome Murphy- O’Connor