This sentence is an example of a cleft sentence (it-cleft, precisely), where the writer has broken down one sentence into two parts, each with its own verb. The writer has opted for this construction for emphasis.
Of course the writer could simply have put it like this: Hitler, and a coterie of his most fanatical supporters, retreated to this place after the Wehrmacht was beaten. But the focus doesn't fall on the place of retreat.
You could read in detail about it-clefts here: