It seems that for assaults on enemy castles, unit scroll is disabled, and you may only select from the units presented on the screen when it initiates. This is a real problem for me, as I don't want to send Stormcaller or Orc Berzerker on raids, and my normal units of choice aren't available (hirelings and rangers).
My work around is to send Stormcaller and Berzerker to storage before I start raids, but, when I get more advanced units, I certainly don't want to be forced to send them. I want to send cheap disposable units, that are quick and cheap to revive.
The bureaucracy is expanding to meet the needs of the expanding bureaucracy.