Keyboard Controlled Overhead Camera Rotation

Hello all,

I'm looking for some assitance with my camera system. Currently the camera sits above a tower slightly further down on the Z and raised on the Y looking down on the tower. I want to be able to use the W,A,S,D keys to rotate around the tower and move up and down on the Z axis. A should rotate clockwise while still facing the turret and staying within the same distance. D, the same, but counter clockwise. W and S should move the camera up and down, while still facing the top of the turret. As shown in this picture: