Veteran creatures.

Sorry, who is Miltia? (Apologies for my slow thinking).
I thought there were Gnomes in Heroes.

Miltia is the first-level unit for humans. They are guys with swords. They are weak, and their morale constantly deteriorates.
In Heroes, there are Dwarves, but no Gnomes.

Added 6 minutes later

In 3 and 4, there's also an analogue, a unit that's been downgraded in level – the Crusader. But in 5, they definitely aren't there.
There is an analogue, but not from the undead. In 1 and 2, the Demon's unique ability to halve enemies was given to a new unit – the Djinns.
In 1, there are no Archmages. In 2, they were more or less present, but then they became weaker.

Yes, I forgot about the Crusader. Then, the Swordsman from Heroes 5 – he's certainly not like the mighty Paladin, but he clearly originates from the Crusader in 3.
I thought about the Djinns (and the Death Knights from 5), but they have too little in common with Demons. Only the ability.
In 1, there are just Mages. Maybe they were created independently of the Castle Building...,

Wendy I learned about it not so long ago – two of my friends immediately installed the ported Castle Building on their PDAs, and now there's something to do during several pointless lectures. The combat system is slightly different, but the controls are incomparably more convenient than in the original. I recommend it to everyone with fancy devices :D