Lowered walls are used by some AI, and are quicker to knock down than stone walls. Of course they can used to make walls Assassin proof by placeing them next to normal walls.
In Stronghold 1 you create a lowered wall placing one of the larger boulders in it. After that delete the boulder and flatten the ground. This halfs the wall and you can repeat this how many times you like.
Lowered walls are used by weak AI's consisting of The Rat, The Snake and The Pig(Sometimes The Wolf uses it but he uses it to great use). Though being somewhat weak and having small resistance to Mangonels they are cheaper and more efficent if you just want a quick form of defence.