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I created a weapon for the upgrade and determined what abilities the units will have.
The first and second archers will be at the level of simple crossbowmen, but without the "accurate shot" ability – in general, they will be ordinary ranged units. However, the alternative upgrade will be special: with a devastating attack, high defense, accurate shot, and maneuverability. In other words, it will be extremely powerful in the early stages of the game, but later it will be better to take the first upgrade to eliminate enemy archers.
And finally, I came up with a good reason for the absence of firearms in other factions: dwarves extract saltpeter for gunpowder from special mushrooms! No need to dig through manure! (You really have to be crazy to conduct manure experiments in a fantasy world).