The castle was enlarged considerably in the fifteenth century.

这城堡在15 世纪得到了相当大扩展。

It occurred on the morning of the fifteenth.

事情发生在15号上午。

The palace was built in the fifteenth century.

这宫殿建于五世纪。

Did the matter in question occur on or about ten o'clock on the fifteenth of June?

那事是在6月15日10点或10点左右发生吗?

August the fifteenth.

8月15号。