I've been searching for this Japanese yaoi manga for ages T_T Its setting is in ancient Japan or China (?) and it's an old manga. Maybe you can imagine the drawing style like mangaka Shin Mizukami or Sakuragawa Sonoko.

It was about uke (an assassin or important person from the enemy's nation?) who tried to kill seme (emperor) with a dagger in the middle of the night when seme was asleep, but he failed. After that, seme forced uke to crossdress and act as his concubine as an imprisonment.

If I remember correctly, seme treated uke well. Uke would have conflicts with other concubines because seme favored him. At some point, seme gave uke a pet bird. When uke and seme hadn't confessed each other's feelings, uke had anxiety to the point of he thought the bird represented seme, so when the bird tried to fly away, uke felt like seme would also left him and chose other concubines.

Uke almost fell from a building to catch the bird, and seme saved him. I didn't remember if seme made uke his empress, but seme only loved uke and they had happy ending.

Sorry, English is not my native language and if you remember the title, please kindly inform me. I will be very grateful :")