The physical sill or doorstep at the entrance of a building or room.

doorsill

a flower in a cage, a crane in a basket; an idiom referring to a person who has lost their freedom

A cart with a cage, used for escorting prisoners.

cage (for animals)

bird in a basket, monkey in a cage; a prisoner or person who has lost their freedom

to be inundated with visitors; to have a doorstep worn down by a constant stream of people calling at one's home or office

railings of a balcony

railing, fence, or banisters, typically in classical architecture

carved pillars and jade doorsills; a heavily decorated, luxurious environment

cloud-wreathed windows and misty thresholds; a tall building with windows high in the clouds

To speak the truth with unwavering courage or to fearlessly remonstrate against authority, referencing the Han Dynasty official Zhu Yun who broke a palace railing while demanding justice.