Yeah caps aren't really an immediate problem since I find it very unlikely that we'll never want to get three or more basic minions... Upkeep would be a problem since it would slow down our rate of growth permanently.
Though I guess that since growing the dungeon costs mana while also increasing mana income there is somewhat of a permanent cost to using mana on minions... But OTOH minions are also likely to be one of our main ways to earn mana by killing or delaying intruders. And presumably we need them to protect ourselves which is also rather critical.
By the way right now my line of thinking on how to make this dungeon produce maximum mana is based on this bit:
If we can capture or otherwise delay intruders we might be able to get a lot more mana from them than simply killing them (though IMO that should certainly be an acceptable alternative). A room with a trap that makes intruders fall asleep could be a big income boost and if it can keep them asleep until they starve to death it is even better. And IMO that sort of thing fits the fae theme perfectly.
@Metrux So, two questions/comments about your system here.
One tied to what I just said, how would something like what I just said be resolved? What kind of rolls determine the effectiveness of something like a sleep effect (or any disable or debuff for that matter)?
Secondly I'm considering the rules as you've written them for the combat system and I think there might be a complication you've failed to take into account... Suppose our dungeon gets invaded by Gravebringer, Angel of Bloodshed and Slaughter who has attack 10. And he runs into one of our pixies which has defenses 1 and life 3. So Gravebringer tries to kill our pixie... And he rolls only one die (with +9 on it granted) because our pixie only has one defense and so he can only inflict one damage each turn. So it takes him three (or more) turns to kill the pixie. This doesn't seem to be working as intended since this would mean that in a lot of case having very low defense is actively helpful since it caps how much damage an enemy can do each round. Nothing can kill a pixie in less than three rounds without special rules. Are you sure that is how you want things to work? It might be a better idea to simply always have dice rolled equal to the attack score and then give the defender a penalty for having less defense than the attack in question.