"While I do like Persians, an interesting tribe to choose is the Scandinavians. Their special unit, the Berserk, is a very powerful attacking unit (attack strength: 6).
My strategy usually is:
1. expand as large as possible in the beginning 2. stabilize your towns/cities 3. attack a weaker enemy with a massive invasion of Berserks.
4. enjoy the fruits of a larger country

Its best to attack with a diverse group of units that way a sudden swift clashing of forces will not wipe out all of your weak defensive units (+2 Berserk defense). Protecting them by having at least 1-2 strong defense units.

Overall though the most important concept of the game is acquiring valuable resources: iron, saltpeter, horses. With these you can attack weaker countries without them. Acquiring luxuries are important too, you can trade them for cash and techs as well as keep your people happy." - idiocyinc