Behind our house there lay half a mou of vacant land. 

此句因there 而倒装。lay (lie) 是不及物动词，类似be 。原句等于说：

Behind our house, there lay  half a mou of vacant land. 

There lay half a mou of vacant land behind our house.

是因为Behind our house这个地点状语，而there无实际意义，有没有there，这句话都要倒装。

= There，behind our house,  lay half a mou of vacant land. 

给网友一些材料看看：

这个句子倒桩是因为前面的Behind our house还是 there？

赵振才《英语常见问题解答大词典》有：

    大家都知道，一般的地点状语位于句首并不要求主谓倒装。例如，In the nursery thechildren were playing happily but noisily.(孩子们在托儿所玩得很欢乐，但很吵闹。)但是，当句子的主语传递新信息时，就常常采用主谓倒装的语序。其目的是为了“把听者心中所无的事物暂且捺住，先从环境说起，然后引出那个未知的事物：总之，是要把已知的部分说在前，新知的部分说在后，由“‘熟’而及‘生’”(吕叔湘语)。

只有在这种情况下，地点状语位于句首时，主谓语才实行倒装。例如：

1. In the garden lay the old lady. (CGEL)

在花园里躺着那位老妇人。

2. There at the summit stood the castle in all its medieval splendour. (CGEL)

在顶峰耸立着一座闪烁着中世纪光辉的城堡。

Behind our house there  lay  half a mou of vacant land. 和

Behind our house lay  half a mou of vacant land. 的区别

In the garden there was a sundial.

    Since the place adverbial In the garden provides in itself the condition enabling us to position the subject after the verb，there is no grammatical requirement for there to be present：

In the garden was a sundial.

    It would seem that whereas the AVS construction without there is motivated by the wish to achieve end-focus...”(CGEL，P.1409)

    这后一句话，人们选用没有there的AVS (状语+谓语+主语)结构， 原因是希望达到句尾信息核心的效果。