Diplomacy, for recruiting creatures of levels 1 and 2.

Let me remind you that expert diplomacy gives a 30% chance of recruiting neutral creatures, not counting other modifiers. Out of 9 guards, only 3 will join, and if the creatures are native (belong to the same castle as the hero), the probability of recruitment increases, up to a maximum of 5 out of 9. However, you can't build a large army this way, as not everyone will join for free.