Some units are able to be unlocked through faction upgrades, such as Singing Scythe and Whaler, but there is not clear indication on the actual unit that they need to be unlocked first. At first I thought that the unlock just meant you get to ignore the Crown requirement for the unit, but Whaler doesn't have a crown requirement. I don't know if I am missing something, or if I am miss understanding something, or if I am expected to just look through the unlocks and note which units require unlocking.