Another issue that I don't think it has been specifically covered above, is the trading rep loss caused by destruction of the AI civilisation that you have a gpt trade with. I used to make large gpt payments to AI civilisations on the brink of destruction in return for technology, so as to try and save a lot of money (i.e. I might only need to make 1 or 2 payments out of the 20). However I have stopped doing this as when such an AI is destroyed I suffered a large trading rep loss. A way round suffering such rep loss is to keep the ailing AI alive by giving them one of your cities for free. If you have a city you can give them then you should consider first selling all its infrastructure and pillaging all its land improvements (Just to ensure it remains weak. I have had a case of doing this and then finding that another one of my cities culturally flipped to the "weaker" civilisation). You then should ensure you get a ROP with it and station defensive troops all around the city (on all 8 tiles and perhaps 2 or 3 troops per tile) so that the other AI civs can not destroy this last city. Then you can wait and milk the AI (whose city you have surrounded) for all the technology it has (I am playing in Deity level so I am always behind in technology). Once the ailing AI has nothing more to give you, wait until the ROP is over and then destroy it.