mage tower robbing:

i had a powerful wizard once, he was immune to fire and negative energy. He had some resist/immunities to other stuff because he was multiclass.

But there were things he was totally not immune to. Like cold.

Now, lets assume for a minute he was your wizard in the tower.

And your thief was a spell user/magic item buyer, and he went in there to set up a cold trap, like a bear trap tied to a cone of cold.

There's no legitimate reason for my powerful wizard in his tower to not get frost bite. He would survive the cone of cold, because frankly, he had lots of hit points.

But he wouldn't be immune. As DM, i wouldn't handwavium some "anti frostbear trap protection field" into existence. I hadn't thought of frost beartraps, so why would they suddenly exist in my universe just to spite my player's creative ideas?


And thus you should run all scenarios.

it is what it is. If it's clever, and you didn't plan for it, neither did the NPCs.

if the NPCs are insanely old or insanely wise/intelligent (like max character mental stat or above, 1000+ years old, etc.) you can make a single "maybe they would have planned for this sort of thing" check that you, a normal human being might have missed. But don't make that check easy.

If they have the wisdom of Athena, maybe its 55-75%, but if they are merely a Genius, don't give them better than 50/50.