Banners and Logos
I'm thinkin' we could have banners to identify which player controls the regiment. (if the units are in squads, which i hope they are) These banners could be customized from pre-set colors, icons, and borders to give a small look of customizability to your army. The banners could also flap from your towers and hang over a newly conquered enemy.
Now, im not sure if customizability is a word, but this would be so cool, and work very well in a game like this, where it is based in a time where every castle is a different lord and a different army.