Bill said it was a railway gun. I said it wasn't. We're both right. It was moved by rail (how else, especially back in the day?) but was assembled into a purpose-built emplacement. The emplacement was concrete, and took weeks to construct. They were substantial enough that they still exist. [Interesting article on several large german guns here.] There was a central pintle for the gun (which answers Trias' question about training the gun for direction) and, as can be seen in the picture above, used little railcars to move the projectiles and powder from the preparation site to the gun proper, where they were hoisted to the firing deck by means of the crane.

Just take a look at the number of men it took to crew these guns - and wonder if those things weren't resources better used elsewhere. And obviously, not a weapon of maneuver warfare. Of course, it's a lesson the Germans didn't take to heart, what with the Dora 80cm gun they used (for, oh, 45 rounds or so) during WWII. The one that took 2000 men to crew and 26 trainloads to move around, and two weeks to assemble. And shot 45 rounds...