Apparently, these are crusaders.

External dwellings of useful creatures.

Fireball answered absolutely correctly. If there are any questions or suggestions on this topic, please write them here (though I will be appearing online quite rarely now, so it is entirely possible that I won't be able to answer for some time (so if someone who knows the answer responds—that is only welcomed))

P.S Invisible - external dwellings are simply dwellings (external houses, creature spawners), and by "powerful" it doesn't necessarily mean level 6 or 7, but rather the external dwellings of those creatures that have high piercing ability or a lot of health (the damned crusader swordsmen (special property—they hit twice, good damage for their level), the damned vampires—these external dwellings, although they are level 4, are considered a very decent supporting force for early capture of Dragon Utopia, cans, and nagas; further—minotaurs, cows, trees, thunderbirds—these are level 5 monsters, a stack of these creatures is a great asset in Dragon Utopia thanks to their bulk (high HP per single unit) and good damage dealt, as well as the special properties of some of them (especially—wood (trees) and cows (often, having a good stack of the latter—using only them as meat shields can break Dragon Utopia)))