Hiring monodrones is usually cheaper. They are the simplest Modrons, and they would be perfectly willing to work for a Lawful king (the evil-good axis doesn’t come into play) because it increases the amount of order in the multiverse. But every modron has Truesight.

Hell, maybe hire a whole team of modrons. Monodrones to stand watch at all ingresses, with orders of “raise an alarm if you see any disguised shapeshifter enter through that window / door / arrowslit / whatever”, and duodrones with orders of “patrol the castle and raise an alarm if you see any disguised shapeshifter”.