Making a game unrealistic is not exactly the same as dumbing it down, only making it easier to learn. Take chess. All the rules of chess are basically arbitrary, transparent (every player is expected to know the rules before playing), and simple, yet we've been playing it for centuries. If you want to see other games with arbitrary, transparent, and simple rules that manage to pull off what I would consider deep strategy, try Battle for Wesnoth (it's free!). Just because a game has simple rules, that does not mean it has only simple tactics.
Now with my little sister explanation, I'm not implying your little sister is dumb, just that she would be unlikely to be interested in medieval weapons and armor and thus less likely to bring to the table any preconceived notions of what weapon should do what. If having those preconceived notions makes you better at the game, this flaws the game by
A. making the game less accessible to people who aren't medieval weapons fans
B. making the game less predictable because different people believe in different realities. The devs might think one thing must absolutely be true and you think another and you go "WTF! That's not even real!" (Try it out. Go to any forum you suspect will have medieval geekery and ask whether longbows can penetrate plate armor or whether a European knight or Japanese samurai should win in a fight)
C. making the game less predictable because computer games are never programmed perfectly. You might see some esoteric rule of real warfare play out in-game and think that must be a bug... or a bug play out in-game and wonder whether that really would happen in realistic war.
On the other hand, rules that are arbitrary, transparent, and simple helps a game by
A. making it equally accessible to everyone (though a smarter person may access it more thoroughly)
B. making the game completely predictable, allowing players to analyze the rules and come up with tactics based on the information the game will actually be using to calculate results.
So no, less realism is not the same as dumbing down a game.
Now, if I were to roughly propose a vision of how weaponry works in this game, it might be thus:
Weapons are loosely categorized into a couple of traits.
Light, medium, and heavy
hand weapons, pole weapons, short missile, long missile
Different weapon materials (iron, mithril, wood, etc)
simple, martial, exotic
All weapons are "designed" at a research phase by picking a set of traits which generates a weapon based on pre-defined templates.
Let's say I pick light, pole, wood, martial. The game would define that as a wooden spear. Exchange martial for simple and it's a sharpened stick. If I want it to change the material to iron, it becomes an iron-tipped stick. Changing the light trait for heavy would give me a scythe, or to medium for an iron spear. Changing the pole weapon trait for hand weapon might give me an iron sword.
For each faction, weapon traits could end up being different, or they have special weapons when using certain sets of traits, like a certain nation might never have access to crossbows (metal + long missile), but can still rely on bows (wood + long missile). Maybe a certain weapon using metal, exotic, and heavy becomes a fearsome faction-specific lajatang while all other factions using the same combination get battleaxes. Another nation might be the only in the game that can choose to have blessed weapons vs. non-blessed weapons.
The soldier himself would have a similar chart of traits
Foot, horse, pegasus ()
No armor, light armor, medium armor, heavy armor
Conscript, regular, elite
unshielded, round shield, tower shield
5, 10, 25, 50, 100
(armor and shields may belong to yet another category)
Again, different combinations of traits get different kinds of units... I don't see how these traits aren't self-explanatory. Also again, different factions might get exclusive traits or bonuses for certain traits acting together. Maybe a barbarian faction has the traits "sane, berserk" or another sort of faction can pick conscript, regular, elite, and then heroic.
Now, every trait mentioned here has its own bonus or penalty, with the exception of faction specific combos that have special traits, too. For weapons, the lighter a weapon is, the less damage it can to do, but the cheaper it is to field. Different role weapons have different bonuses and penalties, like long missiles can shoot farther, but short missiles can shoot faster or be cheaper or hand weapons do bonus damage while polearms always strike first or can strike from a tile away. Units can be customized to your liking, but at all times there are trade-offs involved with every type of weapon and every type of soldier.