Constantly clear out monsters on the map
You would need to clear out the monsters as often as possible to get resources - try to fight the biggest one that you can fight without suffering heavy injuries. Once you have enough levels, join your allies in fighting the more dangerous threats on the map like the Undead Giants.Develop and Recruit heroes
There are a lot of heroes that you can Recruit, and while most of them are useless, you would still need a bunch to improve your kingdom. This works the same as upgrading techs - try to focus on production at first then switch into military later. Focus your resources on developing your top combat heroes so that they can lead your troops - fighting enemies would be much easier that way. Train your rarest heroes first - blue and green heroes are useless later on so don't focus on them.