I was recently looking at the spell Wall of Stone and I was thinking about whether or not it would be possible to create an almost impenetrable cube using this spell.
Here is my math:
In the spell description it says “The wall can have any shape you desire” and "you can crudely shape the wall to create crenellations, battlements, and so on." I want my wall be in the shape of a 5 ft by 5 ft cube without a bottom: that is, it has a ceiling and four walls but no floor, since the spell description says it must “merge with and be solidly supported by existing stone” we'll have it connect to the stone on the ground.
Then we consider how many hit points it would have: We have “ten 10-foot-by-10-foot panels” to work with. Each panel can be any shape we wish, and could be seen to be made up of four 5 foot by 5 foot panels, so we could consider that we have 40 5ft×5ft panels that are each 6 inches thick. If we divide those 40 panels among the 5 sides of our cube (being the four walls and ceiling) each side can get 8 panels. 8 panels × 6 inches per panel × 30 hp per inch equals 1440 hit points per side, and it would seem you get 7200 hp for the entire cube (in case for some reason your enemies decide to not concentrate their fire on one side).
Would this work? Am I forgetting any rules that say I can't do something like this?