"Balance of Terror" showed us the weapons control room, with the weapons crew reporting that forward, midship and aft weapons were all ready. From this, we can logically extrapolate that the Enterprise had forward, midship and aft phasers and torpedo tubes. Further, a repeated clip of background intercom dialogue used on the Bridge during numerous TOS episodes did include a report from "Aft Phaser", implying these was at least one aft phaser bank and/or control officer reporting in.