1 9 7 2 , ▁and ▁a ▁dream ▁about ▁Japanese ▁school ▁children ▁who ▁are ▁waiting ▁for ▁Santa ▁Cla us ▁when ▁they ▁are ▁attacked ▁by ▁inv ading ▁al iens . ▁N ish ik ado ▁himself ▁has ▁c ited ▁At ari ' s ▁ar cade ▁game ▁Break out ▁as ▁his ▁insp iration . ▁He ▁aim ed ▁to ▁create ▁a ▁shooting ▁game ▁that ▁featured ▁the ▁same ▁sense ▁of ▁achiev ement ▁from ▁comple ting ▁stages ▁and ▁destroy ing ▁targets , ▁but ▁with ▁more ▁complex ▁graphics . ▁The ▁game ▁uses ▁a ▁similar ▁layout ▁to ▁that ▁of ▁Break out ▁but ▁has ▁alter ed ▁game ▁mechan ics . ▁R ather ▁than ▁b ounce ▁a ▁ball ▁to ▁attack ▁static ▁objects , ▁players ▁are ▁given ▁the ▁ability ▁to ▁fire ▁project iles ▁at ▁moving ▁enemies . ▁ ▁Early ▁enemy ▁designs ▁for ▁the ▁game ▁included ▁t anks , ▁combat ▁plan es , ▁and ▁batt les hips . ▁N ish ik ado , ▁however , ▁was ▁not ▁satisfied ▁with ▁the ▁enemy ▁movements ; ▁technical ▁limitations ▁made ▁it ▁difficult ▁to ▁simulate ▁flying . ▁Hum ans ▁would ▁have ▁been ▁easier ▁to ▁simulate , ▁but ▁the ▁designer ▁considered ▁shooting ▁them ▁imm oral . ▁After ▁the ▁release ▁of ▁the ▁ 1 9 7 4 ▁an ime ▁Space ▁Batt les hip ▁Yam ato ▁in ▁Japan , ▁and ▁seeing ▁a ▁magazine ▁feature ▁about ▁Star ▁Wars , ▁he ▁thought ▁of ▁using ▁a ▁space ▁theme . ▁N ish ik ado ▁drew ▁insp iration ▁for ▁the ▁al iens ▁from ▁a ▁novel ▁by ▁H . ▁G . ▁Well s , ▁The ▁War ▁of ▁the ▁World