It forces you to keep only the good ones i guess, upgrading is a part of the game
Aye upgrading is the fun part of it
A little pet peeve of mine with the game is its impossible to organize the order of your units. (Well, unless one sell-capture-and-breed the units in an order.)
I like having my girls & gobbos being catalogued in a neat & orderly fashion. Its just pleasing for the eye when one open the units tab. Plus it also brings immersiveness to the game, like story-wise speaking you truly are managing a goblin tribe & its army
Having that.. err--bug?/feature?--well, lets say a secret extra slots 'options' - outside the 30 captive limits on past versions gave me the trick to do just that. (You can create a blank slate hybrid that you can utilize to 'copy-paste' your monster without having them leveling up.) *PS. Though now is unnecessary keeping your monster level low due the Dungeon in V.0.68.9.2.1 no longer levels up with your highest ranking monster -- I still like having my monsters level standardized. And I like having a way too to just save & reload-that-save to force labor the stuck-out preggo on the stirrup -- Creating that blank slate hybrids that I either use to move and neatly line up my monster unit-order -- or just sell her without losing the good ones in my slots.*
Cuz outside the aesthetic purposes, greed or whatever ocd reasoning it was

-- I also learned that the game capped your gold, around 2 billion now it seems, and selling your units past that point will zero out your money. So I still try to keep my monsters not leveling up too much. My rule of thumb is best be with pretty numbers, enough to solo every area, & still making a big sum when I sell it too (so they can still fund me on buying & ordering potions - but not that big enough to zero me out.)
This is my cave and its soldiers. I try to collect every color and each unit types.
- That first goblin on the monster tab is my one & only remaining monster from the first batch (pre-Brothel & Dungeon era/he's level 345 & has been there since v.0.68.4-ish)
- The rest of the monsters are second-gen batch. (I cap them all on level 1769 when Laab & Dungeon was introduced -- with most of them yellow goblin original models are the direct offspring of each unique-named girls.)
- The last unit on my monster tab is my money maker, with the Dungeon pose no more threats in raids Imma level him up higher so I can easily bank gold & funding on standardizing the rest of my girls with potion traits.
This is how my gobbos idle after a raid, I find it cool that they choose to sit and organize themselves that way

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I hope Lodos know he's making a good game here. And I hope he and his team will be interested to develop somekind of a story & lore for mudblood any day soon. Letting us fill the game with our own imagination is fun, and it will be far more engaging when it is paired with the game's world building too.