There have been many reasons given for the assault on the Golden City. It would seem to have been a consistent belief that the Golden City was the realm of the gods/Maker, so whether you were seeking power, immortality or just trying to find out why the gods weren't answering you, that would be the logical place to go. Even the elves believed that the Golden City was the realm of the Creators. It is also noteworthy that in Drakon's vision of the end of the world, he describes the Black City as having 7 gates. That would in fact be one for each god. Is this what mages normally see when they look at the Black City, so he could have learned this from one of them? Otherwise, it is odd that he would say there are 7 gates. In the Canticle of Silence (author Hessarian) there is only one gate, just as there is only one Maker, although in Threnodies 5 (Andraste) it does speak of the Maker sealing the "gates" of the Golden City.
If Corypheus was responding to a promise of power from Dumat, did the latter urge him on because he wanted breaking free from his prison or was he deliberately urging his servant to his doom? Was the throne of the city empty because the Maker had left or because the gods had been sealed away? Did the Magisters going there cause it to turn black or was the corruption already there and the shining city merely an illusion that was shattered the moment they touched it?