I have won a few of the Real time Strategy games without air power and none without walls.
The AI opposition will target anything you need for victory. Walls are a cheap protection. Build enough walls early on to secure production/resources of your military units, and you can go steadily from 10% to 99 % of the map.
If it worked for ancient Rome, Mussilini’s Libya, all of those RTS games, I think it might be a good idea in Afghanistan & Iraq.