A prison in the shape of a cage can be up to 20 feet on a side and is made from 1/2-inch diameter bars spaced 1/2 inch apart.
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The wizard used it as a cage (meaning it has openings throughout the cage)... Which the creature would have total cover, but all attacks would go through the cage. The Cage prevents the creature from escaping the cage. They could attack, if they have ranged attacks through the cage just like the PCs but probably with disadvantage or cover since they'd be attacking through cover as well.
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The PCs all took to using cantrips for up to the 1 hour duration to just cast a bunch of spells at the monster. I deemed it took around 15 - 20 minutes to cantrip it to death.
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Thoughts on how you've balanced this spell? Or is there any clarification on whether monsters can be killed while in the cage?
A few things:
1) "A target with total cover can’t be targeted directly by an attack or a spell, although some Spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle."
- If you deemed the cage to provide total cover, your players should not have been allowed to target it at all. No cantripping to death.
Fireball would've been fine.
2) "A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three--quarters cover if about three--quarters of it is covered by an obstacle. The obstacle might be a
portcullis, an arrow slit, or a thick tree trunk."
- Since you allowed your players to target the creature, did you factor in +5 AC/+5 Dex saves? Personally, I'd also tack on disadvantage to attack rolls as the
forcecage's bars are denser than the average portcullis.
3) You specifically mention liches. A
force caged lich being targeted by paltry cantrips would first cast
globe of invulnerability, then
counterspell whatever high level spell the player would use to overcome the
globe, then
disintegrate the
force cage.