为什么要倒装15.Beneath the streets of a modern city ____ of walls,

倒装用于here,there,now,thus,then + 动词 + 主语的句型中（谓语动词多为be,go,come等）.
Here comes the bus．汽车来了.
There goes the bell.铃响了.
Now comes my turn.轮到我了.
以out,in,up,down,off,away等副词开头,谓语动词是表示“移动”的go,come,leave等句子里.
In came a stranger in black.进来了一位穿黑衣的陌生人.
Down fell the leaves.树叶掉了下来.
注意在完全倒装的结构里,如果主语是人称代词,则用正常语序.
Out she went.她走了.
Here we are.我们到了.
表示地点的介词词组位于句首,谓语动词是表示“存在”之意的be,lie,stand,exist等句子中.
South of the lake lies a big supermarket.湖泊的南边是一个大超市.
On the floor were piles of old books,magazines and newspapers.地板上是一堆堆旧的书报杂志.
Beneath the streets of a modern city 就属于表示地点的介词短语位于句首的情况,所以要倒装.