Use 'byzantine' to describe structures that are excessively complicated and difficult to understand. The tax regulations are byzantine and require an expert to decipher.
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Metaphoric Usage
'Byzantine' can be metaphorically applied to situations or systems, not just physical structures. The plot of the novel is as byzantine as the ancient city it's set in.
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Negative Connotation
Often has a negative connotation, implying something is overly complex in a frustrating way. Navigating their customer service was a byzantine ordeal.