at one time 可以放在句子的哪个位置

The city at one time must have been prosperous, for it enjoyed a high level of civilization.

想问下这句话中at one time 都可以放在句子的哪个位置，可以放在been之后吗？

还有must have been 是推断的使用方法吗，可以详细讲解一下吗，谢谢~

The city at one time must have been prosperous, for it enjoyed a high level of civilization.
【翻译】这座城市一定一度非常兴盛，由于它曾拥有高度的文明。

at one time 虽是介词词组，但它是副词使用方法，意为“过去；一度”，跟 once 同义。其地方应该非常灵活，句首，句中，句末均可。放在be 后也可以，见后面的例句。

Alaska was at one time connected with the tip of Siberia.
阿拉斯加过去和西伯利亚的东端连在一块。
He was at one time so devoted to piano playing.
他过去那样沉迷于弹钢琴。
My younger sister was at one time a boarder.
有段时间我妹妹是寄宿生。

must have been 表示对过去十分一定的推断，没第二种意思。不容怀疑。