Gregary the "Smiling Friend" mage is really good at Charm Person, and his Mystic Aura is overwhelming. His tower is surrounded by a shantytown of people who got lost in his eyes and never returned. Most people assume this makes him a very poor target for adventurers- Nobody wants to tangle with a mob of fanatics, many of whom would be dangerous opponents on their own, let alone as part of a dark menagerie.
The village has two weaknesses, though, that a clever party might exploit to murder or plunder Gregary. The first is that they must follow his orders. The second is that constant mind-alteration has permanently changed most of the residents, and they'll wander around making declarative statements about whatever is on their mind, which often includes their orders.
Thus, in between outbursts about how comfy and easy to wear shorts are, the gatekeeper might mention that they must raise and lower the drawbridge every time the bell rings. Because this is a magical order, they must lower the drawbridge every time the bell rings, regardless of whether that's wise or helpful to anyone. The obvious application is to throw rocks at the belltower to come and go freely. Alternatively, just ring the bell over and over to work the gatekeepers to exhaustion.
I'm working on thinking of more examples, but I'm very proud of the core idea.