Well, traditionally, angels are messengers, and their wings represent the power they've been given to perform their duties. Chaining them up negates their power and prevents them from delivering their message. Where you go from there depends on whether you want to interpret the symbol in a positive light or a negative one.
A positive interpretation could be something like a past revolution, where the (fallen) angel represents a corrupt power that manipulated people with lies and propaganda. The fact that he's chained up and struggling, rather than dead, could be a caution against allowing future corruption to take hold in society. (Or at least this is the narrative that those in power want people to believe--it may or may not be the truth.)
In a negative interpretation, the angel could represent a legitimate and righteous force that was deprived of its power and prevented from fulfilling its duty. Like a royal family that was ousted and escaped into exile (chained up). They've built up a new civilization in a new land, but they're still hoping to someday return to their rightful place (still struggling against the chains).
Either way, the effort and resources required to put symbols like that all over, plus their placement in an imperial palace of all places, means that they're likely related to some kind of political narrative that the imperial family and/or government are trying to push. Unless you want to make them all the work of one particularly eccentric emperor, in which case they could mean anything or nothing at all.