wall of sand

U.S. troops build a wall of sand around an Iraqi town: "U.S. soldiers fed up with almost daily bomb attacks on their patrols near Iraq's main oil refinery are taking drastic measures to fight their shadowy enemy -- they're walling in an entire town."

Something there is that does not love a wall,
That wants it down. — Robert Frost

They know what they are walling out, the insurgents, with this fortress.