Before the Black Gate

[Mithadan]

During a re-reading of LoTR, I was struck by a few passages in the chapter "The Passage of the Marshes". Late in this chapter, Tolkien describes the lands before the Black Gate. This is some of his most descriptive and vivid writing, and likely hearkens back to his own memories of the trenches in the French no-man's land (indeed he uses this very term, "Noman-lands") during World War I. He writes, in part, as follows:

The gasping pools were choked with ash and crawling muds, sickly white and grey, as if the mountains had vomited the filth of their entrails upon the lands about. High mounds of crushed and powdered rock, great cones of earth fire-blasted and poison-stained, stood like an obscene graveyard in endless rows, slowly revealed in the reluctant light.

Frodo, Sam and Gollum hide behind a mound of slag and later below the lip of a pit, where they evade detection not only by the sentinels who walk upon the buttress over the Black Gate but also a Nazgul which flies overhead. In short, Sauron's "front yard" is unkempt, filled with mounds, pits, craters, hills and ridges, each of which represents a potential hiding place for an enemy. My question is why?

Why does Sauron allow the lands before his Gate to be filled with potential hiding places? Would it not make more sense to reduce these lands to a flat, featureless plain which would allow anyone crossing it to be seen from miles away? In fact, this "Noman-land" allows Frodo, Sam and Gollum to escape detection and pass on to the south and west of Mordor where they eventually enter the Black Land. It seems to me that allowing his front yard to be so unkempt is a gross error in strategy that runs contrary to all military reasoning.