16!

Forcecage, Level 7 Sorcerer/Wizard spell.

Forcecage is a handy spell for harmlessly imprisoning a target for a period of time. The spell lasts for 2 hours/level, but considering it is a level 7 spell that means at least 26 by the time you get it.

Nothing except maybe air can get through, though it does block gasses and breath weapons. I think it is just handwaved in the rulebooks to keep you from suffocating people ;P

It's also normally 'invisible' but we decided to have it just barely perceptible instead.

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