We enter the mosque from the side of the propylaeum or Roman Gate to what formerly was the Temple of Jupiter. That by itself had been erected on the spot of the earlier Temple of Hadad. By the time the Umayyad mosque was built it contained the church of St. John the Baptist. In that church the muslims had a place of prayer (there was a habit of sharing places of devotion). A piece treaty that had been signed with the Christian community seventy years earlier had denied the caliph to confiscate the ground or have it sold compulsorily. So he bought the whole site from the Christians on a voluntary basis. The church was demolished leaving an empty space of 157 x 100 meters, bounded by the Roman walls of the temenos (containing the altar, temple (if any), and other sacral or natural features, such as the sacred olive in the temenos of Pandrosos on the Athenian Acropolis; Enc. Britt.), which were broken by the propylaeum and to the east by a lesser entrance.
I quote from Hillenbrand's book Islamic Architecture. He goes on to claim Al-Walid, who had the mosque built, was well aware of the effect of "physically superimposing his mosque on the ruins of a pagan temple and a Christian church [so] he was asserting that Islam had superseded earlier religions.
Well, that will do for an introduction. You can see part of the propylaeum olutside, we now enter properly.