Bibor
Doomsday Machine
Sorry for being so harsh - was having an off day. But as others have stated building wealth and science are the best mechanisms for expanding windows of opportunity, barring stuff like wonderbread and failgold. They're also the best mechanism for sustaining your economy, whether in space or conquest games. The metagame that you mentioned was considered the norm maybe 5-10 years ago, but now it's almost indisputably out.
I'm not the best player by any means so don't take me as the absolute authority for this but if you're not willing to take my word for it then look at some of the most recent fast wins I've done.
Hi there. The metagame I posted was for chill Monarch games I played back then. Obviously it's out for quite a while

I looked at your saves. I discarded the noble games by default, not because it's on noble, but because you can conquer the whole world with 15 knights (exactly as you did), which kinda makes the whole point of building anything other than research or gold pretty moot, as you don't need units and you certainly don't need neither happy or health, as you have them all anyway and your sole aim is to get as much research/turn as possible, as fast as possible.
The deity game was far more interesting. Again, I get the whole point. The capital was carrying the brunt of research while other cities were busy whipping those ridiculous numbers of cuirassers / cavalry until the world decided to break under all that hoof. There isn't much time or point to build anything when you're unit-whipping your cities to the bone anyway.
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Yes, the whole point of building research and/or gold is to compress turns. Why invest 1000 hammers into city buildings when you can have the same amount of beakers/gold?
Well here's the logic. If a market costs 150 hammers, you build it only if you can get 3-4 extra happiness out of it that you would otherwise have no way of getting and that extra happiness can be put to good use.
The return of that investment needs to take into account:
a) what else could've been build during that period and would that other build return the investment faster?
b) how long does it take for those 3-4 pop to grow so the market actually starts paying itself off?
c) once that extra population starts producing, how long will it take to return the investment and start turning a profit?
If 3 extra pop can work 3 plains mines (which is a big IF considering it also requires 6 surplus food that can't be used otherwise & 3 surplus health), that's 12-16 extra base production every turn, so it would take:
cosidering 6F surplus at size 11 w/granary I think the math would be: 4+6+11 turns = at least (probably more than) 21 turns to grow by 3 pop, not counting any health issues.
During this time you'd get around 64 hammers back, so paid off 30% of the investment. The remaining 90 hammers would be hammered out in 7,5 turns for a total of around 30-ish turns.
Considering an existing base production of 10-15 hammers, that would make it 40-45 turns for the investment to start paying itself off.
Now, if you get currency at around turn 100, spend at least 50 more turns acquiring the 3 resources to make it worthwhile, and add those 50-ish RoI turns into it, you're looking at T200 it starts paying itself off.
So, is the marketplace a good investment? It depends! In your Germany game certainly isn't as the game ended by turn 195
