Metaphorical device where one object is substituted for another, related object. Cf 'crown' for monarch, 'bench' for judiciary.
The Sublime Porte. Where did my obsession come from? I think it's probably related to the fact that I explored the delights of metonymy (and its close relation, synecdoche) in my first term at university, around the same time I was writing essays about competing Imperial interests in the run up to WWI. The Sublime Porte seemed to me a glorious shorthand for the Ottoman Empire, much more exotic and interesting than the Court of St James, say. (Or the more prosaic 'Berlin', 'Washington' and 'Brussels' we get today.)
So I was determined to track down the original, actual gate from which the term derived. Easier said than done. The gate now leads to the provincial governor's HQ and is not particularly on the Istanbul tourist trail - though it is in the Sultanahmet district, along with the Topkapi Palace. In fact, we discovered the gate through sheer serendipity. We were walking along Ankara Caddesi looking for a restaurant and I looked up - and recognised the distinctive canopy over the gate.