Prior to Aegon’s conquest, would you say that the area that became the Crownlands was sort of like Poland or Belgium? That is, a flat, open region that lacks natural defended and is regularly overrun by its more powerful neighbours?

Well, that comparison is made more commonly to the Riverlands. 

The Crownlands is a bit more complicated, because there’s very different parts of it: Crackclaw Point is different from the northern coastal centers of Duskendale and Rosby, which are different from the island territories of the Bay, etc.