Hi Hoa Thai, I think this is a difficult question, because the sentence is quite poetic. However, here's my opinion: I think the author has used 'but of' with 'a soaring lightness' because we don't expect the ramparts to be light as they are three storeys high. In more normal language we could say 'despite being three storeys high, the ramparts appeared very light'.
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Neeraj JainHere but of is referring to the ramparts.Hi Neeraj Jain,