It may not hurt them, but it may draw more attention to this area. They will still be able to bark and howl and call for more capable guards. Or more of their own.

[He wasn't saying no to the lightning cage so much as he was saying it might not be the best option if they wanted to maintain a stealthy advantage.

With the size of the place and the number of patrolling beasts, his own ability to make them invisible would only get them so far. And might be better to use on the way out.]