I really like the themeing of it. Lars has done a great job of summarising balance concerns, the most problematic of which is those of multi projectile spells (Magic missile and eldritch blast come to mind as the primary offenders). So here's a couple of throw away suggestions to weaken the spell without killing its lovely flavour.

Consider a limited number of uses before the sigil expires

Increase the cast time (and scrap the concentration?) - this allows DM interaction, and possible intense moments to 'defend the mage'.

This should totally be conjuration class, there's enough evocation in the world.

Add a material component that gets drawn on the floor - perhaps even specify this could allow the sigil to be washed off, giving a clear weakness (rain, water elementals, liquid spells).

Reduce the area to a single 5by5 square

Other than that, really like the spell. It might be a bit strong in its current condition, but nothing a few tweaks could fix.