
Originally Posted by
Bramborough
I think the disconnect is that higher-level buildings don't show the recruitable units from their lower-level predecessors. For example, the Saxons' Level IV keep enables recruitment of Chosen Nordic Warriors, while the Level III version unlocks Hearth Guards. Once the Level IV building is built, its panel does not state "Enables recruitment of Hearth Guards". But of course Hearth Guards are still recruitable in that province, because of that particular building line.
The other wrinkle is military tech development, which upgrades units and makes early ones no longer available. For example, at different tech levels, a settlement can recruit Nordic Levy, OR Nordic Spearmen, OR Elite Nordic Spearmen, but never all three (or even two). Once Nordic Spearmen become available, then Nordic Levies are gone; existing Levies might stay in your armies (if you don't upgrade them), but you won't be able to recruit any more.
The best way to deal with this is to just become familiar with the in-game Building Browser and Tech Tree, and refer to them often. Much better than the truly awful Encyclopaedia (the one area in which Attila is worse than R2...which is saying something, because R2's encyclopaedia was pretty bad in itself).