The Primus Redoubt is what happens when you want to mount a Titan gun on top of a Bunker. Because fuck you that's why!
The Primus Redoubt is a super-heavy emplacement usually built by the Imperium to defend vital locations such as cities or foundries (damn, man, why does anyone even bother attacking them!). Protected by its heavy walls and a force dome, the Primus Redoubt can withstand practically anything thrown at it.
According to the fluff, they can be armed with a variety of heavy weapons that are more usually found on void ships, powered by a generator deep beneath the main emplacement structure. Yet crunchwise it carries a twined Turbo-Laser Destructor. Not weak by any means...but still far below the level of main weapons on an Imperial voidship.
Hilariously mislabelled Primaris in the Index, scunthorpe strikes again! This fortification takes up a Realm of Battle tile to deploy and has a massive titan-strength fuck-off laser. It enjoys being able to actually hit with the thing with BS 4+. Nowhere near as tough as it once was, but also much cheaper in points. At less than a third the cost of a Warhound, it's not a bad way to get a cheap(ish) turbo-laser. It can't really be spammed, though, what with it being a RoB tile.