Going to war with an Army of One is a cute concept, and all... but down here, it's more likely that you'll get by with a little help from your friends. And by friends, I mean Minions. There's definitely strength in numbers! To that end, you're going to need Hatcheries: the more Hatcheries (a 3x3 room that becomes unlocked once your Dungeon Heart has been upgraded to Level 2) you have - and the higher you’ve leveled them - the higher your overall Dungeon's population capacity will be.
To make things MORE interesting, each kind of Minion takes up a certain “allotment” of your population capacity. For example, an individual Skeleton will only use up 1 population point; Trolls use up 2 points; Warlocks, 3 points; and so on. When looking at the Summoning Portal of your Dungeon Heart, be sure to check out each Minion’s individual stats by clicking the blue “i” (for Information) box in the upper-right of their portrait, of the Summon Queue window. The field you're looking for is "Housing Space"; this will indicate how many population points are required for one Minion of the kind you have selected.
So is raising population capacity all the Hatchery is good for? Not quite! Each Hatchery is also equipped with a natural defense: the Eggshell Mortar! This powerful defense does 3x3 area splash damage to your enemies, making it particularly effective against clustered groups of enemies. Talk about getting egg on your face!
In addition to increasing population cap, upgrading Hatcheries also increases the overall HP of the room, and increases the damage dealt by the Eggshell Mortars.
Sounds like an essential component to add to your Dungeon, right, Keepers? Eggs-actly!