It's not really about the effect of closed walls. If walls are used like they are supposed to be used, there's no problem. The real problem arises from people who only want the wall towers, and use them as defense towers instead. They don't have a minimum range (and it's hard to give one without limiting the length of walls), they shouldn't be attackable by regular soldiers, they have a bigger armour, they cost less, ... The plan is to, in some way, bonus the usage of walls like they should be used. With suggestions s.a. non-shooting connectors when you're not using long wall pieces, not letting towers stand allone, ...