D&D 5E - What level is this spell?

That seems a bit powerful when compared against Wind Wall, also a third level spell, along with more utility than Fire Shield, a fourth level spell. The spell seems to want to do a lot of things, and that's understandable considering the original, and I'm not sure if that is really the 5e way. There are spells in 5e that have long descriptions, but those are usually spells that have complicated ending conditions or multiple saves for progressive effects. Then there are the ones that are just complex, like Prismatic Wall/Spray.

I mean the spell A) stops opponents from approaching you (or at least make them pay a heavy price for doing so), B) prevents ranged weapon attacks outright (magic weapons?), C) imposes disadvantage to melee weapon attacks against people in the sphere (but what if the opponent has already 'paid the price' for entering the sphere and is inside?), and D) all weapon attacks (ranged and melee) can be redirected on a miss against a target in range (at the cost of a reaction). Not to mention that you can protect multiple people with this sphere.

I presume it would be on the Wizard spell list? I think this spell, as written and if I understand it correctly, would be a bargain at fourth level.

Yep, definitely a wizard spell. Maybe sorcerer, not sure yet. And conceptually, you may be right - it does too much for 5e. That was my gut instinct, and why I initially placed it as higher than the AD&D 3rd level spell it was in Dragon magazine.

There are two guide posts I'm using to balance it... fire shield (it should deal less damage overall than fire shield ~40) and wind wall (its damage should be comparable or slightly superior to wind wall ~27).

Damage calculations
Wind wall does 3d8 damage in a 50' long x 20' high x 1' wide wall when it is created. Using DMG pg. 249 "Adjudicating Areas of Effect", we can surmise wind wall is expected to catch 2 targets (50 / 30, round up). And 2*3d8 = 27 damage.

Fire shield does 2d8 whenever you're hit with a melee attack. Generally, DPR calculations in 5e are done using a 3-round assumption thanks to DMG pg. 278 under "overall damage output." We can further assume that the caster is subject to 1-2 melee attacks each round (after all, they thought to cast fire shield so they were expecting melee). 3*2d8 =27 damage. 6*2d8=54 damage. We'll take the average and assume that's the amount fire shield was designed toward. (27+54)/2 = 40 damage.

Thinking about your points
A. makes opponents pay heavy price for approaching you: I assumed that one opponent would try to approach you in the combat, taking 14 (4d6) damage, and once others saw that they would adapt to other tactics if capable...if they realize bows don't work, then they'll need to switch to...heavier thrown weapons, spells, or other attack modes like breath weapons. Granted, there are plenty of monsters in the MM with some variant of bite/claw/sword/bow as their only attacks, so maybe you're right this needs tweaking...
B. prevents ranged weapon attacks: To be fair, wind wall does exactly the same thing, and on top of it deals damage and hedges out gases/gaseous form monsters, and keeps small flying creatures at bay! I mean, I literally took the wording of the second paragraph (about preventing ranged weapon attacks) from wind wall in the PHB.
C. imposes disadvantage to melee weapon attacks against people in the sphere: First, it's a reaction and you only get one 1/round. So the caster will only be able to impose disadvantage on one weapon attack per round. Second, imposing disadvantage 1/round as a reaction is actually weaker than shield (a 1st-level spell which grants +5 AC); because of the ability to protect allies, however, I considered it comparable. And you're right that it's meant to only apply to creatures attacking through the sphere, not once they're already inside of it; I'll need to clarify.
D. all weapon attacks (ranged and melee) can be redirected on a miss against a target in range: Again, this is bounded by the 1/round reaction rule. But I assumed that in our hypothetical 3-round scenario that imposing disadvantage would cause 2 out of 3 of those attacks to miss (mathematically disadvantage causes the probability of reaching a target AC to decrease by 1/2 to 1/3). So 2 attacks are actually redirected, and then the attack roll is done again, and I assume 1 attack hits. This begs the question...

How much damage does a monster facing a party capable of 3rd-level spells (i.e. 5th level) deal? There's no right answer, but we can make assumptions. For this example, I used the gladiator (CR 5) from the MM, with a spear attack dealing 11 (2d6+4) damage.

And that reflected attack actually deals extra fire damage due to fire spiral / warding flame, so... 18 (2d6+4+2d6 damage. This puts the damage output of the spell at 32 (14+18). Which is 5 points higher than wind wall, but 8 points lower than fire shield.

What do you guys think? Are my assumptions off base? Should I reduce the damage it deals, shave off a die here or there?

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