How much work you actually put into the thing is up to you, but a hands-on approach will pay off when you're inevitably attacked by enemy hordes. We had a chance to see one of these sieges unfold all Helm's Deep-style, with the hero running around on the battlements and smashing bad guys with the help of his own army. It gets pretty chaotic, too, and you'll have to contend with relentless waves of skeletons and other monsters rolling up in giant, destructible siege towers, all while magic spells and other projectiles rain down at you and explode spectacularly.
Your troops can take care of themselves, but only to a degree - without your help and leadership, they'll be chewed up rapidly, so you'll need to issue orders on the fly, deploy your defenses and throw down defensive spells to give them an edge. And, yeah, you'll have to get in there and tear it up, because that's what heroes do.