How do the phrases “herded together,” “barb of wire,” and “forbidding shapes” contribute to the meaning of the poem?

They emphasize the narrator’s recognition of the grim history the tourists are observing

The quotes are similar to phrases you would hear when people described what happened during the Holocuast. The Jews were "herded together" onto cattle cars and into concentration camps where they were held behind barbed wire. Depending on point of view, the "forbidding shapes" could be an image of the Nazi soldiers or the Jews. In the poem they refer to the continents.