expression, ichidan verb
go down the stairs; descend the stairs
A set phrase meaning to physically walk or move down a flight of stairs. The verb おりる (下りる/降りる) is intransitive, but the phrase uses を to mark the path of movement.
彼は急いで階段を下りた。
He hurried down the stairs.
エレベーターが故障したので、階段を下りなけれ
The elevator was out of order, so I had to go down the stairs.