After some thought, I think the best would to go ballistic on this and give him as much powers as you want, but let it come with a price. If a guy could animate an entire village or city Transformers-style, destroy the entire enemy army and go to luch, it would be a pretty boring book. The good thing is it would be short as well. Instead, he could still have all those powers, but it could be very dangerous to use them.
If you have played many computer games, you have problaby heard about Baldur's Gate. One of my favorite spells there in summoning a powerful demon that can kill pretty much anything in sight without breaking a sweat. The problem is you only summon the demon, but you don't control it. This means it can kill all your enemies for you, but it will treat you as an enemy as well. What if the castle did this? The guy can animate the castle or even village Transformers-style, but he knows prefectly well the castle / village will attack anyone nearby? This again means everyone in the castle / village needs to flee first, and the guy will most likely die.
Another thing you can think about is going medieval-Transformers in a way, so the guy isn't actually doing anything to the castle. Instead, he discovers that the castle itself is a living being. No one controls it, but it can perhaps become an ally? The people inside the castle protects it, maintains it and all that, so if the guy can awaken the castle-creature, it can protect the people inside it and itself?
Either way, I really don't like not using idea because "it's already used". Instead, I like to take an idea and turn it into my own. More or less every single idea you can come up with is already used anyway, so unless we re-use old ideas and turn them into our own, the last book ever written has already been published.
