Why should they be crippled? It's better for a Horcrux not to draw attention to it, apart from maybe the diary which was intended to be a weapon. Voldemort does guard his Horcruxes with dark spells, but if he'd used Hogwarts, that wouldn't be necessary. Unless combined with other spells, a Horcrux is just a part of a soul bound to an object, and unless you have a special Horcrux tracking devise, I don't see how you could find out. It wouldn't even make Harry's scar hurt, seeing it didn't cause him any trouble when he was dealing with the diary. And Voldemort had access to Hogwarts, that day when he went asking Dumbledore for the job (which DD was sure he didn't want or expect to be given). It was then that Voldemort had to realize he wouldn't be able to ever search Hogwarts for items relating to the founders, so what would be more plausible than take the castle itself instead? He was able to perform magic unhindered and undetected on the Hogwarts grounds, as you can see by the curse on the DADA position. Tom Riddle was more attached to Hogwarts than he's ever been to a person. In HBP, it says
Hogwarts was where he (Tom) had been happiest; the first and only place he had felt at home.
Harry felt slightly uncomfortable at these words, for this was exactly how he felt about Hogwarts, too
Not only does it show Tom's feelings about Hogwarts which makes it likely for him to consider making it a Horcrux, but also hints at Harry's very own dilemma should he ever be faced with the need to destroy Hogwarts. A hero's sacrifice, anyone?