Archer balancing
Archers are extremely powerful in early game, some say they are overpowered. I wouldn't go that far, but I do think they need some balancing.
I suggest implementing a researchable shield (a large shield) that reduces effectiveness of archers against that unit (circa roman legion, best layman example is from the beginning of the movie gladiator). I wouldn’t go so far to say immune, but maybe halves the damage from archers or doubles the armor against archers only.
Also, the Yithril Empire faction starts with archers (actually only 1 tech level away). I think this is extremely unbalanced. Other empire and kingdoms are 4 – 5 tech levels away, with the % success of research this can end up 6-7. Depending on the presence of local libraries, this can be a long time (50/75 turns). Yithril starts out with this tech, in the beginning of the game there is very little to counter more than 1 archer unit. Suggest examining this balance, maybe reduce the number of tech levels to get to archery for everyone or extend out Yithril research giving them some bonus to archers late game to match the Master Archer race benefit.