The idea is that all generic units are unupgradable However everyone has access to all the unique units (except samurai). Most unique units will not be created in castles units like the Huskarl, Teutonic knights, Condeiterro, and Obuchs would all be created from the barracks. The only unique units that would be created from castle units will be those that defy classification like Flaming Camels and War Wagons, or hybrid units like Ballista Elephants, Throwing Axemen, and Gbetos. Unique units that are direct upgrades to Generic units like Imp skirms, Imp Camel, Houfnice should be all researchable at an increased price to account for the price of the elite upgrade.