I AM THE STRATEGY GUIDE
But seriously, any questions you have I would be happy to answer.
Keep in mind that having one city for the first 50-100 turns is a good option as the fewer cities you have, the faster they will grow. 2-3 cities by turn 50-150 is a well rounded strategy. Just make sure the tile you settle on has 4-6 grain if you want it to be able to get to level 5.
You almost always want your Sovereign out in the field battling monsters. Find a few heroes to go along and you should be able to level rather quickly. Remember that the more units you put in an army, the more total XP you get. Ex: A Bear gives 50XP. Attacking with one unit gives that unit 50XP. Attacking with 6 units gives each unit 50XP.
As far as combined arms, you seem to get it already. I have never used a catapult, lol, didn't think they were in this version. Heroes are currently always better when out fighting. They can lead an army or group together, up to you.