Complex sentence: After the restoration of the monarchy in 1660 all of the City gates were unhinged and had their portcullises wedged open, rendering them defenceless, but they were retained a visible sign of the prestige of the City.
Split into simpler sentences: After the restoration of the Monarchy in 1660 all of the City gates were unhinged and had their portcullises wedged open, making them useless for defence. They were retained despite their uselessness for their original purpose because they were a visible sign of the prestige of the City.