I'm kinda torn on this, should you be able to level up your sovereign/adventurers at a later date i.e. save up levels and spend them all at once.

Or be forced to level up instantly after battle upon acquiring the necessary experience.

Good stuff

If you save it up you can say see where you need to place your stats on a sovereign. some games i want more mana to draw from, other games i want more intelligence, and i need to explore a bit to see how the game is going to be to decide this

or you only have 5 essence and your spent it already imbuing, now you just hired another adventurer you want to turn into a caster and instead of hiding them in a city you can level them up and imbue them once you have the essence to do it without losing any potential state gain on essence.

Abuse

I only really see potential areas for abuse (feel free to tell me more if you see them) HP and surprise casters out of no where.

HP: your adventurer barely survives a battle but wait you have 3 levels saved up, so you pump 6 points into hp and gain that much back for the next fight.

Caster: Right before a tough battle you realize your out of mana and your going to need it for the next major engagement of the war but never fear you have an adventurer with 5 levels saved up. a simple imbue later and you have a very powerful and caster ready to go(though  don't remember if you start with full mana or not that could make this moot)

I'm leaning towards allowing saving up levels but i think the casters should start with only a percentage of there total mana capacity, they did just gain the ability to tap/control it.