Inferno tactics.

Well, you forgot about other heroes and their initial units?;) First, second - external dwellings.... There can be quite a few of them.... In fact, demonology is an extremely complex matter.... but if you know how, you can gather a lot of demons....
And regarding further development - I don't think Efreet and Devils can be turned into demons.... So they remain, along with Pitfiends and demons - is that not enough?;) Higher levels.... You can do so many things - it's scary!

I'll break it down =)
We're not talking about the whole game... you can gather 1000-2000 there.
This is about the beginning (at least the first month). Look:

1st week - you start - resources are at zero.
On the first day, you barely manage to build a Prefect's Office (and that's not a given), maybe you beat someone if you're lucky.
Second day - you run around and collect something, build at least a Market.
Third - you build a Guild, start fighting with someone.
4-5-6th days - you build everything for Pitfiends: Magogs, Guild 2, Demons....
7th day - you build a Stronghold... so that you have at least a few Pitfiends and Demons....
RESULT: You won't have a single coin to buy all of the above + (as you say, to engage in demonology) and buy 2-3 more heroes... that's 5000-7500 gold... and you definitely won't have enough money to hire from external dwellings (even if they exist) even in the second week, because you simply don't have a Municipality.
I can definitely say that "demons and Pitfiends" - that's not enough!!!! even very little =) if you cast Slow on the demons, 50 Archers (from the Castle) will defeat 100 demons, even if the level difference is twice as much... unfortunately, that's how it is =(

Regarding the "phrase that Efreet and Devils cannot be turned into demons":
and if they could, would you turn them into demons???:confused:
I haven't met anyone who has even tried to do this :smile42:

Added 10 minutes ago

no... impossible, impossible, everything is quite possible... in the first week, Imps and Demons go with whips. We try to upgrade them in the second week as quickly as possible. after which, the Imps are divided into 9-18-27 (to resurrect 1-2-3 demons, respectively). So many with whips... they kill 9 Imps, you revive 1 demon and send him back... in this way, you can get 3-4 demons in every battle in the second week, 6-8 after the increase with whips =) and so on. I don't remember the exact number of maximum demon resurrections... in this way, let's assume that we resurrect 5 demons from 45 Imps. 3 slots for whips, 3 slots for Imps, 1 slot for demons
Imps in battle... they are killed, we revive them and send them to the demons, it's not bad to have another hero with Imps accompanying such a hero.... in total, 15 demons per battle, 20 battles, 300 demons... on large maps, in a long game, and with the right approach, you can really do a lot,.... building a Castle on other mobs doesn't make much sense compared to defense... because compared to a hundred demons, 10 Efreet look weak.... + don't forget about the demon increase....

Now look: 9+18+27 = 54. It's logical, but considering that 54 Imps is one and a half increases. =) What kind of "every battle you can get 3-4 demons in the second week" ??? there won't be enough Imps =)
P.s. the excuse about heroes from the tavern doesn't work, because you definitely won't have enough money to buy them =)