What suggestions and hints are given that David’s world is an unusual world (e.g. different from our present day world)?

When David shares his dream about the city with is sister, she tells him there are no citieslike he's seen in his dream..... that possibly, he'd had a dream about the olden days.... the days before the Tribulation.

When I was quite small I would sometimes dream of a city—which was strange because it began before I even knew what a city was. But this city, clustered on the curve of a big blue bay, would come into my mind. I could see the streets, and the buildings that lined them, the waterfront, even boats in the harbour; yet, waking, I had never seen the sea, or a boat....
And the buildings were quite unlike any I knew. The traffic in the streets was strange, carts running with no horses to pull them; and sometimes there were things in the sky, shiny fish-shaped things that certainly were not birds.
Most often I would see this wonderful place by daylight, but occasionally it was by night when the lights lay like strings of glow-worms along the shore, and a few of them seemed to be sparks drifting on the water, or in the air.
It was a beautiful, fascinating place, and once, when I was still young enough to know no better, I asked my eldest sister, Mary, where this lovely city could be. She shook her head, and told me that there was no such place—not now.