The Cenacle and the Tomb of David are located in the same building; it has a minaret and small cupola and stands in the shadow of the great round church of the Dormition (built 1900).
The C4 reconstruction was first known as ‘the Upper Church of the Apostles’, and then in the C5 as ‘Sion, Mother of all the Churches’. Since it is impossible that the niche behind the Crusader cenotaph could have belonged to a synagogue, as some claim, it is best to see it as a receptacle in the exterior wall of an inscribed apse. The stones were blackened and cracked by the fires which consumed the church of Sion in 614 and again in 965. The building owes its present form to the reconstruction by the Franciscans in 1335. The arches of the upper room are typical Lusignan or Cypriote Gothic.
The tradition concerning this building as the site of the Last Supper is unreliable. It is first attested in the early C5 AD and appears to be a derivation from the better supported tradition which located on Mount Sion the descent of the Spirit on the apostles at Pentecost (Cyril of Jerusalem, before 348). This latter event took place in an upper room (Acts 1: 13; 2:1), and it was natural to assume it to be the same one in which Jesus ate his last meal with his disciples (Mark 14:15).
Source: The Holy Land by Jerome Murphy- O’Connor