Walls were proposed barricades for Command & Conquer: Generals, just like in the other Command & Conquer games. They were removed from the game, presumably because they needlessly slowed down an otherwise fast-paced game.
There are various straight and corner pieces for walls for all factions, as well as selection pictures and tool-tips. Presumably, the walls would be built using the removed line build feature, which allowed the player to build buildings along a straight line. Each faction has a specific type of wall; according to the tool-tips which are left in the game, they were as follows:
United States: Laser wall section. It blocks vehicles from passing and triggers an alarm if enemy infantry tries to pass it.
China: Traditional concrete wall, which just plainly blocks troops from progressing.
GLA: Burning barricade. This one does not really block anything, but the fire damages the units, if they passed by.