It might also help you to look at the problem in terms of 2D instead of 3D. Taking out the height/altitude component in your angle calculations reduces the problem to simple trigonometry.

It might help you to look at the problem in terms of 2D instead of 3D. Taking out the height/altitude component in your angle calculations reduces the problem to simple trigonometry.

So the basic algorithm becomes this (pseudocode):

// Assume Up is <0,1,0>
Vector2 target_position( target.X, target.Z ),
        tower_position( tower.X, tower.Z );
// Quaternion construction using axis and angle.
double angle = tower_position.angle( target_position );
Quaternion tower_yaw( Vector(0,1,0), angle );
// Then concantonate the current rotation with the rotation you calculated, or you can interpolate with a slerp function if you wish.