I have a mental picture of a royal court, a large table, spread with maps, the king and his counsellors poring over these very valuable documents, and Isaiah standing quietly in the corner, waiting for instructions; "will someone fetch me the map of Persia?"
I don't know if this is remotely likely to have ever happened. It's probably more likely that Isaiah was part of the inner circle of the Temple, more allied with the priesthood than the monarchy, but sometimes can detect in his thinking that birds eye view of events that ordinary people probably never had.
Nowadays we can all watch the news and see a picture of the globe zooming in to a particular country, city, district, street, house, to allow us to understand where in the world this newsworthy event is taking place. In Isiah's day, many people would never travel beyond their daily horizon more than once a year.
In chapter 14, we have the long view. We are watching from the satellite, standing over the maps in the throne room, looking into the future. Assyria is the big threat right now, but Isaiah is looking beyond them for most of the chapter. There are signs that there is a new empire rising - Babylon - and one day it may eclipse Assyria. But Isaiah is seeing beyond even that day, and describing the mocking taunts that will accompany Babylon's fall.
Babylon, that hasn't finished rising as of yet, will inevitably fall. That thought brings comfort to the counsellors: never mind today's threat, tomorrow's threat will pass as well. At the end of the chapter, the fall of Assyria is almost mentioned as an afterthought, and as the picture zooms back in to the present, we realise that we are a turning point for Judah - Ahaz the king has died, and his son Hezekiah is untried as yet. The Philistines are jubilant - they think they've seen off an old foe - but this long view reminds Isaiah and us that it takes more than the death of a king to interrupt God's plans.