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Indeed. Perhaps it will help by adding extra vertical force to the cells when they are at the sides for 3 seconds.
 (either into or out of the sides will do, as they will bounce back) .

Not saying that the invulnerable spaces are not good. 
(It adds strategic considerations of allowing cells to get through the defense line or not)
But letting cells stay there for 5+ seconds without any signs of coming out can be frustrating