Before BtS I had just moved from noble to prince in Warlords and I was playing quite good.

Nowadays I keep constantly losing in BtS on noble, which is getting frustrating.

What am I doing wrong? Is there someting in BtS AI I should know to get even?

Yes, even though BTS proclaims to be "Beyond the Sword," it is perhaps even more Sword dependent than Warlords. Do not neglect your military! You want to establish the most POWERFUL civilization in the game.

Also, it is important to check your advisers often. Become familiar with the adviser windows, and it's not a bad idea to know the hot keys to your advisors. For example, the foreign advisor is F4, and check this nearly every turn just to see what techs the other AIs have. Another important advisor is the information one, which I forget what key it is, but I think it's F8. Pay close attention to your power meter and do not leave yourself vulnerable to the AI's attack.

Also, if you're not already familiar with this, it is important that you specialize your cities. It is better to have a few strong military cities that are not as good at research, a few strong research cities that are not as good at production, etc. than to have a bunch of average cities.

I tend to feel that the "beyond" part of the title indicates chronology rather than importance.... more like "after the sword".

Interestingly, I personally find BtS a lot easier. I think that the AI in BtS was designed to counter a specific style of player (aggressive/rush style) and it is very well designed to achieve that.... however, not everyone plays like that.

I personally find the AI expands slower, techs slower and generally wastes too much commerce on Espionage.

The net result is that I grab a lead both geographically and technologically.... and I tend to keep that lead throughout. Even if I start in an awful position, I can reel in the AI over the course of the game.

Just giving you a different perspective on it.

If you want some advice from the folks here, perhaps you could outline your general strategy?

My first advice is always - choose a leader you like and play with them until you win. :) Learn to leverage their traits, UB and UU and it will give you more understanding of what you can do in the game.

Ah, I have some difficulty expanding at monarch - Generally because the other civs have better starting locations - I usually cap out at 9-10 cities on a huge map. So long as you keep a fair sized standing army you shouldn't have problems with early age AI aggression.