The temple complex at Baalbek was intended to become the largest place of worship and religious ritual in the entire Roman empire. Given just how large many of the temples and temple complexes already were, this was an impressive undertaking.
Before Caesar instituted his plan, though, Baalbek was relatively unimportant -- Assyrian records have nothing to say about Baalbek though Egyptian records might. The name itself cannot be found in Egyptian writings, but Lebanese archaeologist Ibrahim Kawkabani believes that references to "Tunip" are actually references to Baalbek. If Kawkabani, then it looks like the Egyptians didn't think Baalbek was important enough to even mention in passing.
There must have been a strong religious presence there, though, and perhaps a widely regarded oracle. Otherwise, there would have been little reason for Caesar to choose this place to put any sort of temple complex, much less the largest one in his empire. There was certainly a temple to Baal (Adon in Hebrew, Hadad in Assyrian) here and probably also a temple to Astrate (Atargatis) as well.
Construction at the Baalbek site took place over the course of nearly two centuries and it was never really finished before Christians assumed control and ended all state support for traditional Roman religious cults. Several emperors added their own touches, perhaps in an effort to more closely associate themselves with the religious cults here and perhaps also because over time more and more emperors were born in the general Syrian region. The last piece added to Baalbek was the hexagonal forecourt, visible in the diagram in the above image, by emperor Philip the Arab (244-249 CE).
an integration of both the Roman god Jove and the Canaanite god Baal, images of Jupiter Baal were created using aspects of both. Like Baal, he holds a whip and appears with (or on) bulls; like Jupiter he also holds a thunderbolt in one hand. The idea behind such blending was apparently to convince Romans and natives to both accept each other's deities as manifestations of their own. Religion was politics in Rome, so integrating traditional worship of Baal into the Roman worship of Jupiter meant integrating the people into the Roman political system.
This was why Christians were treated so badly: by refusing to even offer superficial sacrifices to the Roman gods, they were denying the validity of not just Roman religion, but the Roman political system as well.