NCF – Extended list of Neutral creatures.

Hmm, that's an idea. It would be cool, of course, but in a game, some players might be able to recruit seven types of powerful creatures (especially Dark Elementals with teleportation without giving up the turn and returning, and Mind Elementals of level 6), while others won't. There aren't many of them that spawn in a week – 2-4, depending on the type, and the price varies from 500 to 1700. And how did you implement the Hell Gate in RTF? They spawn randomly there.

In RTF, I did it like this: I simply wrote it in the building file:
BUILDING_NAGA_TEMPLE

CREATURE_FIRE_ELEMENTAL
CREATURE_WATER_ELEMENTAL
CREATURE_EARTH_ELEMENTAL
CREATURE_AIR_ELEMENTAL


CREATURE_FIRE_ELEMENTAL
CREATURE_EARTH_ELEMENTAL
CREATURE_WATER_ELEMENTAL
CREATURE_AIR_ELEMENTAL
CREATURE_PHOENIX

DWELLING_TYPE_SPECIFIC

The defense consists of 4 creatures that are always present, and the recruitment offers 5 creatures that spawn randomly.
To make this less of a cheat, you can increase the cost of the Elementals – the income is calculated strictly for castle creatures – 28000. If you make the price of the Elementals high, it will be unprofitable to recruit them, and you will have to change resources and capture mines