A danmaku game where you create and throw "edges" at bosses is a unique yet excellent implementation of the theme and restriction. I really like it, though Edge Knight! certainly reminds me why I just listen to Touhou music. (Speaking of which, the Edge Knight! music has a Touhou je ne se quois, even if it's obviously not up to par with Zun's best work. Still good though.)
I do have one criticism, though. Making the edges your offense and defense has the side effect of making enemy bullets their offense and defense, too—and their bullets are better walls than your walls are bullets. I didn't hit the first non-tutorial boss with a single edge—they were all destroyed by bullets before they got there. There's presumably a way to get through the bullet shields, but I didn't find it.
I'm not sure how to solve this, though; maybe make projectile-edges not soak up enemy bullets or something? Make edges your attack or defense, not both simultaneously? It's a cool concept, but I think it needs some refinement.