Which Castle do you prefer to play as?

In principle, I don't use diplomacy; it's rarely possible to recruit the unit that is actually needed; most of the time, you encounter units of levels 1-3.

+1
Slightly off-topic, but I couldn't resist :-)
Diplomacy is mainly good in campaigns.
For example, in the Yоga campaign, where he can't cast spells.
It's not easy to complete all the scenarios on the highest difficulty level (the last one is particularly difficult in this regard), but diplomacy is quite simple – you just keep attacking all the monsters you encounter, and about half of them join you. By the time you reach the enemy, even though he is a powerful mage, he doesn't have time to tear you apart with spells, and if you drag out the mission, your chances of winning are almost zero.

And in other missions, it helps a lot at the beginning.

If you're playing a scenario, at the beginning, if you have level 1 diplomacy, there aren't many monsters who want to join you, and that's very important. Later, as you develop and upgrade it, everything changes, but you don't really need it anymore. What's the point of having hordes of troglodytes or spearmen in a Rampart army, for example?
Well, occasionally you encounter useful monsters, but it's not a guarantee that they will want to join.
In general, I wouldn't call this skill a cheat (although some people do).
If anything, Earth Magic is the real cheat skill :-)