Question on selling troops

[Warmskin]

I went to Arlania and decided I want to sell some troops to make room for some Marauders. I went to the castle (right near the entrance to the area), but I can't figure out how to sell them... whatever I click on I get no option to sell. What am I doing wrong? And is there a hint guide somewhere that really explains all the things that the manual is lacking (like the actual math used in battle calculations)? Thanks.