Along the wall is watchtowers.

这个句子是个倒装句,正常语序是 watchtower is along the wall.最后的s是表示复数形式,所以与 is 不像对应.把is 变成 are 或者 去掉最后的S.
相关知识
陈述句中出现的几种倒装句
1、 主谓倒装
（1） Long,long ago there was a war between the birds and the beasts.
（2） Once upon a time there were six blind men in India .
（3） “Come along ,then.”said the bird.
（4） …but toward the end there came the terrible storm…
2． 表语倒装
（1） Such was Albert Einstein,a simple man of great achievments（全倒装句）
（2） Inside the pyramids are the rooms for the bodies of kings and queens.
（3） A very reliable person he is.（主语是人称代词时,主谓不倒装）（《英语分类句型》P400）
3． 宾语倒装
（1） Many happy hours she had spent planning for something nice for him.
（2） Twenty-one dollars they took for it .
（3） Not a single mistake did he make .
（4） Only a dictionary he never forgets to bring here .
宾语在前时,多数属于强调部分,句子一般不加助动词,偶尔也有助动词出现,如例句（3）.
4． 复合句中的倒装
（1） I take back what I said .
（2） …and the ship would have sunk with all board had it not been for the captain.要是没有那位船长,就会连船带人都沉没了.
（3） Were I you,I would go with him.
（4） Proud as these nobles are,he is afraid to see me.
（5） Not only was everything that he had taken away from him,but also his German citizenship.
（6） Hardly (Scarcely)had he sat down when the telephone rang.
（7） The harder you work,the greater will be your achievement.
5． 其他倒装
陈述句中为了保持句子平衡或为了强调表语、宾语、状语等成分,或使上下文紧密衔接时,使用倒装句.
（1） On the wall is a blackboard on which were written some words,“It is no use sitting here without listening to me.”（定语从句中,主语的同位语太长）
（2） They entered the classroom,in front of which sat a naughty boy.（使上下文紧密衔接）
（3） Gone forever are the days when the Chinese people were looked down upon by the foreigners.（主语加上定语后太长而表语短,所以表语在前面,强调句子平衡）
这个题应该是表语倒装的情况