"On the narrative side, the prophecy and tagline were more of a reflection of your moral choice, as different playstyles will give different prophecy. Like if you choose to help your units by saving your population or use them as meat shields while attacking from behind. But I guess I made that too cryptic while aiming for a medieval feel, and is something I definitely need to revisit so it makes more sense."
I wanted to respond to this because there's a bit of an issue here I feel: until you unlock the king, there's no difference in play style. You're just making units to try and defend against the inevitable.
With that in mind, I can now see how to correct this problem. First, make the king playable from the start but weaker than what the king ends up being later on. Second, make it that all units are available from the start. Now, shift the upgrades from being static and always available to being more random and based on how many enemies the player defeats (Not enough kills? No upgrade for you). Now you shift these upgrades into three categories: the People, the Enemy and the King. Upgrades about the People will do things like improve your population and reduce the cost for units (doesn't need to be just this). Upgrades about the Enemy will do things like reduce their population per wave and make them weaker. Finally, upgrades about the King will do things like make the King stronger or give and upgrade abilities that affect either the soldiers or the enemy. From here, you can now more clearly define playstyles using the various upgrades.
Another thing I'd suggest as part of this would be to change how waves work. Instead of each wave just coming, make it that a wave has to be completely cleared out before the next can begin. Also make it that the damage enemies do to population if they get all the way to the end is percentage based and that each wave will always have more than enough population to win in a single wave, even if you get all the Enemy category upgrades.
These should help make the playstyle thing work and help shape the narrative stuff better.