What item is preventing gunters from reaching the castle, and how does it work?

From what I can see, you must be referring to the Orb of Osuvox and, by extension, the shield over the castle. In the story, the Orb is an artifact that generates an impenetrable shield over Castle Anorak.
The shield is spherical and can neutralize anything that touches it. Gunters try everything to bring the shield down but are unsuccessful. The usual solutions they resort to (magic and technology) prove useless against IOI's shield.
The Orb of Osuvox is the object that deploys the shield. However, the Orb must be held aloft by a ninety-ninth level wizard. So, the shield over Castle Anorak can stay up indefinitely, as long as the wizard keeps immobile and holds the artifact with both hands (a tall order, but according to the book, this is exactly what the wizard does).
In chapter 34, an SD-03 supply droid detonates an antimatter friction-induction bomb on the observation platform of the castle, where the wizard is sitting with the Orb of Osuvox. The text reveals that Wade reprograms the supply droid when he manages to hack his way through IOI's intranet system. When the bomb detonates, it kills the wizard and deactivates the Orb of Osuvox, bringing down the shield in the process.