I'm about 20 turns in on my Sicily campaign and Venice has decided to blockade Naples. I'd like to use this as a pretext to take the isle of Crete (Iraklion) from them.

My question is, after I take it should I leave it as a castle or convert to city? I just took Rhodes and plan to use that as my castle for crusading. I also have Tunis as a Castle. Is it worth it economically to convert Crete to a city? I'd appreciate any advice. Thanks.