Image from Wikipedia of the Ratte; size comparison with the Maus and the Tiger I, WWII German tanks.
When expecting an invasion along a thousand miles of coastline, it is impossible to build bunkers containing large caliber guns to repel and invasion fleet, every few miles. The Ratte was a design study to build a mobile pillbox to transport large caliber guns along the coast to repel the invasion.
It was not a tank as such. It was intended to move at a few miles per hour to arrive at the invasion beach to fire big guns at ships. The original designs showed no AA guns on it, the idea of Maus AA guns is a post war fantasy, as far as I can tell. It would have been no different than a large pillbox that would be bombed by Allied aircraft prior to an invasion.
That technique met with mixed results on D-Day. Rather than have a large concrete pillbox thirty miles from the invasion beaches, the Ratte could drive at three miles per hour and arrive at the invasion beach overnight. The Ratte would be very heavy and too heavy to drive over anyplace that had not been prepared in advance. No bridge could ever hold it. I suspect they would have been built in modules that would have been assembled in place. Then they would have a special gravel roadway to drive on for fifty to a hundred KM in either direction.
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