From Memory Alpha about the Constitution Class Starship:
Tactical systems Forward phasers of the USS Enterprise
Original configuration Constitution-class USS Defiant firing its secondary hull aft phasers
During the early 2250s, Constitution-class heavy cruisers were armed with a complement of directed energy weapons, that possessed enough power to destroy half a continent in a concentrated bombardment. In addition, these vessels carried on board laser cannons, capable of operating on energy fed remotely from the ship. (TOS: "The Cage", "The Menagerie, Part I", "The Menagerie, Part II")
By 2266, phaser banks were standard complement aboard this class of ship. A bank actually consisted of a single emitter and its power supply, though it was common practice to fire two banks at a time and refer to it as a single firing. (TOS: "The Corbomite Maneuver", "The Doomsday Machine", "The Paradise Syndrome")
Ship-mounted phaser banks had a range of approximately 90,000 kilometers, and like hand phasers, were capable of being adjusted to stun, heat, or disintegrate visible targets, including objects or beings in space or on a planet's surface at a focus ranging from narrow to wide beam. When only motion sensor readings are available, ships' phasers could be set for proximity blast and bracket the approximate coordinates of the target. (TOS: "Balance of Terror", "A Piece of the Action", "Who Mourns for Adonais?", "The Paradise Syndrome", "The Tholian Web")
In the original configuration, a battery of four to eight forward phaser banks were located on the lower part of the ventral side of the saucer section. Aft firing banks were located above the shuttlebay on the secondary hull. There were also port, starboard and midship phasers. (ENT: "In a Mirror, Darkly, Part II"; TOS: "Balance of Terror", "The Paradise Syndrome", "Arena", "Friday's Child").
It is not clear where the additional banks mentioned in background dialogue and in several episodes were located. The top view graphic of the original Constitution-class configuration, used on a computer screen in Star Trek III: The Search for Spock, places port and starboard emitters on the dorsal surface of the primary hull. Two emitters cover both flanks.
The diversion of all but emergency maintenance power to the shields had the adverse affect of reducing phaser power by fifty percent. (TOS: "The Tholian Web")
The Constitution-class originally mounted at least six torpedo launchers, one tube covering the aft firing arc. Forward tubes were located in the same area as the forward phaser banks. The aft tube fired from the end of the secondary hull. This combined arsenal was powerful enough to destroy the entire surface of a planet. (TOS: "Arena", "A Taste of Armageddon", "Journey to Babel"; TAS: "More Tribbles, More Troubles"; ENT: "In a Mirror, Darkly, Part II")