Yes you can use Castellans as your Hero, but keep in mind they are limited in that sense. They have no Might and so cannot call anything heroic. They have Will but can't cast any spells and must spend Will every time they are in combat, so they risk being worn down. And despite their own decent Courage they can't call Stand Fast to help anyone else in the army. So they can fill the role of "leader" for purposes of army composition but will not be a very good choice for actually using in that role.
Just be sure to give them the Morgul Blade option. For only 5 extra points, if you're already sinking that many into a single "almost but not quite a hero" model you might as well have the extra threat it brings. A Castellan or two with Morgul Blades are enough to make almost any enemy Hero very, very nervous.
