Please recommend a time-travel novel. The male protagonist is a prince who plays dumb, and the female protagonist is a eunuch who protects the male protagonist!

The female protagonist seemed to be an assassin or a secret agent, while the male protagonist was a prince who pretended to be stupid and was not favored (in fact, he was very powerful). The female protagonist transmigrated to that era. She disguised herself as a eunuch and stayed by the male protagonist's side to protect him. She even endured the whip of another prince for the male protagonist. Later on, the male protagonist trusted the female protagonist very much and asked her to help him conquer the world. In the end, the female protagonist fell off the cliff and transmigrated back (without amnesia). The male protagonist missed the female protagonist and also transmigrated to the modern era (amnesia). It seemed that he had become the boss of a gang. They met each other. The male protagonist recovered his memories and they were together.