I've been wanting to put a long write up listing all the common mistakes I see lower level players make over and over again in this forum, and hopefully be of some help. It's the top X reasons because I will probably add more and edit this post a lot. After the list, I will go into each mistake in-depth. So here it is:
The top X reasons you can't beat Noble (or Prince, or Warlord, or Settler, or even Monarch) (in no particular order)
1. You don't prioritize developing your capital from turn 1.
2. You work a lot of unimproved tiles.
3. You don't expand quickly enough.
4. You expand too quickly.
5. You don't develop your economy.
6. You don't build enough military.
7. You build too many buildings.
8. You build too many wonders.
9. You found Buddhism or Hinduism and you didn't start with Mysticism.
10. You always try to found Buddhism or Hinduism when you do start with Mysticism.
11. You like to beeline to Archery.
12. You don't get a strategic resource early.
13. You don't have a plan.
14. You don't know how to choose good tech paths.
15. You simply must have a religious shrine.
16. You don't think diplomacy is important.
17. You don't chop forests early.
18. You never use slavery.
19. You don't know how to use civics to your advantage.
20. You love the City Garrison promotion and use it often.
If you are beating Monarch most of the time, you probably aren't making most of these mistakes, or at least not often. You also probably realize that not all of these are always mistakes.
The top X reasons you can't beat Noble (or Prince, or Warlord, or Settler, or even Monarch) (in no particular order)
1. You don't prioritize developing your capital from turn 1.
2. You work a lot of unimproved tiles.
3. You don't expand quickly enough.
4. You expand too quickly.
5. You don't develop your economy.
6. You don't build enough military.
7. You build too many buildings.
8. You build too many wonders.
9. You found Buddhism or Hinduism and you didn't start with Mysticism.
10. You always try to found Buddhism or Hinduism when you do start with Mysticism.
11. You like to beeline to Archery.
12. You don't get a strategic resource early.
13. You don't have a plan.
14. You don't know how to choose good tech paths.
15. You simply must have a religious shrine.
16. You don't think diplomacy is important.
17. You don't chop forests early.
18. You never use slavery.
19. You don't know how to use civics to your advantage.
20. You love the City Garrison promotion and use it often.
If you are beating Monarch most of the time, you probably aren't making most of these mistakes, or at least not often. You also probably realize that not all of these are always mistakes.