Well, I've carefully read many of the tips posted in this forum, but anyway...
I've tried several times to play in Regent level by choosing Huge maps and Pangea type continents (in order to be able to negotiate with as many other civs as possible) and the actual conclusion is that no matter what I'm doing, somehow AI manages to build settlers twice as quickly as I can, so by the time I have around 10 cities, most of AI's civs have 15 to 20 and there is no more land for me to expand in a peaceful manner. And AI then starts to accumulate science, no matter what I'm selling them or bying from them (if I'm trying to use a diplomatic strategy), and I see that the only subject where I'm leading is my culture. In all other areas, I'm a loser because. in the time I get 1 tech, most of AI civs get 2 - as a result when I'm entering the Middle Ages, I'm already some 5, 6 techs back. I build libraries (no tech for universities yet, right?), I've chosen Scientific Civilization... HOW CAN I KEEP UP in science?
or maybe just... WHAT'S WRONG?
And that's, as I already said, only the Regent level...
I've tried several times to play in Regent level by choosing Huge maps and Pangea type continents (in order to be able to negotiate with as many other civs as possible) and the actual conclusion is that no matter what I'm doing, somehow AI manages to build settlers twice as quickly as I can, so by the time I have around 10 cities, most of AI's civs have 15 to 20 and there is no more land for me to expand in a peaceful manner. And AI then starts to accumulate science, no matter what I'm selling them or bying from them (if I'm trying to use a diplomatic strategy), and I see that the only subject where I'm leading is my culture. In all other areas, I'm a loser because. in the time I get 1 tech, most of AI civs get 2 - as a result when I'm entering the Middle Ages, I'm already some 5, 6 techs back. I build libraries (no tech for universities yet, right?), I've chosen Scientific Civilization... HOW CAN I KEEP UP in science?

or maybe just... WHAT'S WRONG?

And that's, as I already said, only the Regent level...