Conditions for recruiting units.

You also forgot to mention that if a quest was completed according to the plot (for example, in the main campaign of Heroes 2, there was a branching mission that could be skipped, shortening the path, but if you choose it, the dwarves will always join you during that campaign; I agree, that's a good help, especially if there are many of them and if you train them on a hill – they become very strong in numbers – or in a castle until they become battle dwarves; also, they have a 40% chance that a spell will not work on them, and then a characteristic sound will be heard).