Asking him for his preference on where to put his castle after shrinkinating and un-shrinkinating/waiting for for the shrinkinator to wear off seems like a good idea.
Dr. Doofenshmirtz builds them all the time, and missing some of the underground tunnels would be embarrassing. As a side effect, the doonkleberry bush in front of the tunnel would come with it, and it doesn't risk accidentaly breaking the castle (as much) like using Norm-bots would.
Sure it would cost an Inator, and maybe an action, but it's a much better option than having a few Norm-bots take the castle apart, ship it overseas in a ship Dr. Doofenshmirtz would have to supply (because there's no way any appropriately sized ships would be in Drusselstein), and rebuilding it on the other side.
On the other hand, that also sounds like something he would do, and then slap his forehead when he realized he could have used an Inator to do it.
Also, can't wait to show up Xanatos when it comes to moving magic castles into absurd locations.
Edit: autocorrect seems to think a large group of Norm-bots is called a "hinder"