Here's a little tactic I found last night.
Build some settlers and plonk them in galleys. Go to an island/continent where the enemy controls most of the land. Look for a place where you can build a city -
anywhere is good, even that one square on the corner of the continent which is a hill or jungle. Go and build the city there then talk to the civ that is closest to the city. Put the city on the trade table and ask what they will offer. Often you will get 2 or more techs, luxuries, resources and gold!
Its most useful in the early to middle game and you find yourself trailing in techs. Used best on archepelago/continent maps where the other civ controls the majority of the continent and and you have no hope of keeping or maintaining your new city. Useful to give the job of building settlers to cities which have nothing to build and are capped at 6 or 12 in population.
It might also help if you talk to other civs and try to trade for their territory or world map if you can so that you can identify places to build your extortion cities
Another plus is that even if the other civ has nothing to give you , you can build and let the city stay there until the civ does have something to give you. Just dont let it get taken over by culture before you trade it
