Diplomacy, for recruiting creatures of levels 1 and 2.

As a reminder, 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 faction as the castle), the chance of recruitment is higher, up to a maximum of 5 out of 9. However, you can't amass a large army this way, as not everyone will join for free.