Quote Originally Posted by supadodo
Constantinople can never be converted to a castle. It starts off as a large city.

In my opinion, I would say Aleppo(or was it Adana? I only know its east of Antioch) because it can supply troops easily to both Antioch and Edessa(Only takes a 1 turn march). Aleppo is also a shield to Antioch against Mongols because of the bridges.
Couldn't you exterminate when you take it to kill the population. I am not sure because I don't flip flop back and forth on castle/city conversions. It would be a perfect castle though.