In my opinion, let players pick a faction and introduce meta mechanics to drive engagement.
Let casuals do shit like farm NPC mobs, play cavalry or build siege engines to contribute.
Combat should favor skill, but allow shit players to really dedicate themselves to doing disgusting peasant shit like rolling rocks down hills, pouring hot oil or jabbing pikes into horses. Battlefield has a lot of appeal for that kind of thing by letting you drive tanks and shit.
From what I've seen, casuals aren't scared at all by deep combat or complex mechanics as long as they feel like they can hop in and have fun on their own terms.
Then all you need to make somebody want to learn, is make them care about the outcome of the fight.