Apart from who built it, what kind of a palace would be built where it's dark most of the time? There wouldn't be a nice garden, and few would like to stroll around in a place like that (apart from dwarves). Why build a palace (as in: very beautiful house) were few can enjoy it?
The bridge has built by the Cyrads and modified by the Narsin, but what was its use? Is it a shortcut to reach the northern part of the city? Would anybody really walk all the way up the mountain instead of around? Even it it were shorter, it would be a hard walk. Or is there something in the buildings connected, and while you can reach one just like that, you need the bridge to reach the other? If that is the case, what kind of a place would that have to be, given that it's obviously meant to be hard to reach...?
The underground portion was built by the Cyrads -- There are bits in the book (and in the Gazetteer) that confirm this -(click to show/hide)