Oh I get it!...its called "Illusion Wall" because the effect IS an illusion-considering that when deployed,those dang monkeys do not "stop",rather,they jump right on through!...and the monkey mage has no trouble firing arcane attacks right through and on target...very funny.-K.
Mokeys move with scamper, which is a free move, they don't have to stop for difficult terrain. If the monkey mage can shoot through the illusion it might be a bug. But not if he just shoots someone in a square with illusion.
