The guns are basically a half-sphere with a smaller dome on top, about 50m in diameter, and 25m tall. The power is internal to the structure. Most of the control mechanisms are within the structure, including the repulsor field generators, and the whole thing is protected by an energy shield. They are compact, and each one carries a single turbolaser.
They're going to be hard to affect without getting through the shield.
Proton Torpedoes are physical weapons, so they can get past the energy shield, but the shield is strong enough to deflect the starfighter blasters.