"# Creatures can now be part of a caste, and this may govern many things, such as their profession, appearance, etc."
This needs clarification, is it as awesome as I think?
What are some examples?
Toady is oddly vague on it, or it's that I just don't understand it as much. Basically every creature has at least two castes: male and female. For basically 99% of all creatures this makes no difference (save for the obvious). However, for antpeople, Toady decide to go all out with castes, such as soldiers, queens, etc., who can use different symbols and various other nonsense.
It's actually straightforward, although concrete info has been a little scattered. Basically, castes are subtypes of a given creature. It's very simple and very powerful. They can have different bodies, different behaviors, be eligible for different entity positions (king, guard, etc.), have different graphical tiles, be immune to different poisons, whatever. There are some restrictions on what can vary, though:
The number of tokens that are creature rather than caste specific is very small. Creatures handle the overall name, biome, whether the creature is considered fanciful/doesn't exist (for art), vermin information (so, no, Fault, sorry!... too much depends on that right now), whether groups hang together closely, whether it is mundane (from real Earth for age name purposes), population information including good/evil/savage, if it is equipment, sphere information (not sure how long that'll last, but there were annoyances), and that looks like it (they also have some general categories that have caste overrides, like child names and speeches for adv mode). Everything else, including body information, is held in the castes.
Also, I doubt there's any requirement to have at least two castes. Most creatures will have male and female castes, but I think the genderless creatures in the current version, e.g. bronze colossuses, will become single-caste creatures.