Algodoo

Nisheeth's contest, my entry.

It spawns a solid and very fast-moving wall of boxes.
The rainbow-y thing is just to draw your eye to the fact that it's actually moving.

Hmm. I realized that a quirk of this entry combined with a quirk of my entry to Destroy the Castle means that this would probably not completely protect the castle from that gun.

WARNING: THIS IS A LAG-BOMB!