User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-2219564-20130202110630/@comment-5379655-20130204071613

Even if they were retractable, a direct hit from a Dreadnought is like a nuclear bomb, but with better penetration. It would likely be really cost-prohibitive to construct a bunker for it rest in as well as the cannons themselves.