Matthew Poole and continuators on Jeremiah 51:12

12 Set up the standard upon the walls of Babylon, make the watch strong, set up the watchmen, prepare the ambushes : for the LORD hath both devised and done that which he spake against the inhabitants of Babylon.

Some judge these words spoken to the Medes declaring the Will of God, that they should use all probable means to conquer Babylon, or (as some would have it) display their Banners upon the Walls of it, as signs of its being already conquered: but certainly it is more reasonable to conclude them the Prophets words to the Babylonians, either rousing them out of their security (for it appears they were strangely secure from Dan. 5. Historians tell us that the City was fortified by Walls fifty cubits high, and 2 hundred cubits broad, and by a very deep and large ditch, besides that on one it had the River Euphrates) or at least quickening them to make all the preparation they could, though all would be to very little purpose, for God had resolved upon what he would do upon Babylon, and it was already as good as done.

Marginal references: Heb. liers in wait.