Gun Turrets were indeed a complex and arcane technology, the manned ones eventually giving way to barbettes, operated from a central fire control position, to eventually disappear entirely in the West with the removal of the tail-mounted 20mm M61 Vulcan Cannon from the B-52H in the early '90s. As Danny points out, a major disadvantage was their weight and the moment from the CoG, especially given that their most effective location was in the extreme tail of the a/c.
For anoraks and historians alike, a fascinating thread on the Lancaster's turrets is here:-
Lancaster Turret