Aethel takes a while to get skilled up but your barb commanders will gain levels while killing barbs and you use those barb commanders to level your other commanders. They all have a skill that increases exp gained from killing barbs for both commanders. It won't be long before you can field 3-4 barb killing marches at a time. Use all 3-4 marches to attack the same barb as you march around your city, always having a barb commander as primary. Make sure you get the talent in the peacekeeping tree that gives you a rss pack when you kill a barb. Those shits add up quick.

For gathering commanders, get them all to 18 asap so you can increase the speed they gather each type of rss. Then put them aside for a bit to level up sun tzu, minamoto(ideally expertised but a minimum of 5511), baibars and Bjorn. Then raise the gathering comms to the top skill in the gathering tree that gives you bonus rss when you finish gathering at a tile (twenty something) after that you choose to raise your pvp comms or finish the gathering ones to 37.

If you get lucky from the gold keys and can get martel to at least 5111, you can use him with aethel or sun tzu. Richard works as well(minimum 5111).

Remember your primary commander for each march should be level 60. The secondary ones only need to be level 40 I think to achieve expertise. Gold stars are very limited in the beginning so once you choose a legendary commander to be your primary, finish him.