Why is Jerusalem referred to as “the valley of vision” when it was located on a hill?

Out of the context in Isaiah 22:7–11, we know that the prophecy is about Jerusalem. However, valley of vision seems like a strange title for a city located on a hill. It may be that Jerusalem was seen as a valley in contrast to the mountains that surrounded it. It may also have been called the valley of vision because Isaiah received his revelation here. Lastly, it has been suggested that Isaiah was a bit sarcastic in his naming of Jerusalem, as the people showed a lack of vision, despite the many visions they received through the prophets.