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Quite the interesting discovery, there are other games with similar gameplay outside of Goblin's Burrow?
Possibly without multiple routes to unlock everything.
You probably don't have enough of something. Maybe living space for newborn monsters. Or resources - each ordinary monster except goblin needs a resource specific to that monster, while heroes all need their own resource to remain fertile after mating.
Apparently, no. He can have sex for pleasure (and apparently, he does it a lot), but to get monsters for fighting, he has to breed prisoners with monsters.