Jamieson, Fausset and Brown on Isaiah 21:5

5. Prepare the table--namely, the feast in Babylon; during which Cyrus opened the dykes made by Semiramis to confine the Euphrates to one channel and suffered them to overflow the country, so that he could enter Babylon by the channel of the river. Isaiah first represents the king ordering the feast to be got ready. The suddenness of the irruption of the foe is graphically expressed by the rapid turn in the language to an alarm addressed to the Babylonian princes, "Arise," &c. (compare Is 22:13). Maurer translates, "They prepare the table," &c. But see Is 8:9.

watch in ... watchtower--rather, "set the watch." This done, they thought they might feast in entire security. Babylon had many watchtowers on its walls.

anoint ... shield--This was done to prevent the leather of the shield becoming hard and liable to crack. "Make ready for defense"; the mention of the "shield" alone implies that it is the Babylonian revellers who are called on to prepare for instant self-defense. Horsley translates, "Grip the oiled shield."