Turn off vassals. It also seems to be easy to get dogpiles going against him if needed, but I never seem to have him threaten victory in a military fashion. And if he does a lot of warring in the midgame, then he will not keep up in the space age tech race anyway.
Are you cottaging them? I aim for river starts as 1st priority, but ample food is critical too. I'd always use flood plains as farms, since I rely heavily on representation specialists. You are never going to be using US/FS anyway. Best to have a couple of corn/pigs or several 5 tiles. And of course plenty forests. I usually try to get forest spread on regular grass/plains tiles, so avoid roading them.
Another thing, that the northwest FP locations often lack, is a large amount of forests outside the BFC. Mass chopping those really helps getting all the critical buildings out faster (NEpic/Oxford in particular). Whenever there are no critical things to build, then the workers should be prechopping those forests. Again, avoid roading open tiles, you want as much forest spread as possible.
Stone/Marble is great too, preferably in the 3rd ring.
Gold/gems is critical for the early push to CS (bonus for slingshot, but I didnt manage to yet, thats the bad thing with Mansa).
I have been cottaging FP's. I have also experimented with cottages in favor of forest preserves on a river tile (more beakers for mature cottage than for sci specialist, but I suppose the GPP's of the specialist make up for it?). I can't see any way to deal with the happiness/health before Drama and Biology with all that excess food. Anyhow, I've tried it every way I can think of.
I will try to attach a saved game here. At this point in the game I am ready to push towards Computers for Internet. I have played out from here 4 times and lost the game in a different way each time. If yo uwonder why so much military it is because Ragnar and Isabella have been PITA's.
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First variant: just push for tech and mind my own business. I get internet, first to Fusion, anyhow tight space race with Mansa, but -- Mansa kills or vassalizes everyone and wins by diplo vote as my spaceship is on the way.
Second variant: I gift his first rival (Sitting Bull) superior military tech. This slows down the war but the Mansa block is surely going to win if it continues. SB votes "no" to stop war resolution and Mansa votes yes, but the war continues. Next turn they make peace. Mansa has captured all of Ottoman lands and re-vassalizes Spain. Diplo game over.
Third variant: I really aid several of the AI (not just immediate neighbors) with gifts from an early stage with military techs for cash. This slows his military and he instead focuses on space. I would lose the space race here since he gets easy trades with them... except that someone (not me) sabotaged his stasis chamber. I launch with 2 thrusters (18 turns to landing) since as soon as he rebuilds he can launch with 5 thrusters (12 turns to landing). But he never rebuilds it because he decides war is a better bet. He does not succeed getting the vassals in time. Instead, Ghandi's 3rd city goes legendary with 5 turns to spaceship landing.
OK... Fourth variant: Sort of an attempt at fixing the immediate problem with the 3rd variant, I just start upgrading everything to rifles, and plan to raze Bombay before doing like as in the third variant. Result - As soon as Ghandi sees me with more than 3 rifles, he voluntarily vassalizes to Mansa. At this stage I think there is no way to win the game... but I forge on. I attack Ghandi... fail to raze Bombay, eventually get peace with Mansa just as his cavalry arrive. Meanwhile Ragnar attacks me with inferior units and dies and takes peace. Right after that, I have assembly line and Isabella attacks me with 18 artillery and 18 rifles. Conquest defeat.
Please note I HAVE attempted this with vassals off (several times) and the end result is that Mansa (or someone else, usually Suleiman) gets realy really big by taking over so much territory they can easily tech fast enough to enable a spaceship while I am building the last few parts, then build an entire spaceship in about 6 turns and launch.