A very valid concern. With more playtesting, balance could be achieved. I was mostly figuring that an attacker would naturally go for movement-based pieces and/or long-range killing. I had not considered a lack of LOS being a block to charging, honestly. It's not a way I've ever seen things played - charges around blocking terrain aren't uncommon. The intent was for the Nightmare Zone to be melee-only, a way for the less-powerful defender to flank the opponent without giving them free ranged attacks. The attacker could then make the strategic choice regarding concentrating units in the gap or sacrificing points to the enemy to spread out.
If I were to edit this going forward, I would likely make the field 5 feet long instead of 6.