Click the triangles next to Edit
Mode. Click Object Mode.
Switch to Front View by clicking
View and then Front View.
The pink line in this picture is your castle wall. You
should notice that it is way to high.
Press the G KEY move your mouse down so the wall moves down.
Right Click the
triangles next to object mode. 
Click Edit Mode on the menu that pops
up. 
Press the A KEY twice to select all vertices. All vertices should be yellow.
Press the E KEY. Click Region.
Move your mouse up to create
your wall.
The last thing we need for our castle is a floor. Press the A KEY.
Press Shift S. Click Cursor-> Selection.
Press Spacebar > Add > Mesh >
Circle. 
Set the vertices to 5 and click Ok. Set the vertices to 5
and click Ok.
Press the S KEY and scale the pentagon to make the floor of
the castle.
Press Shift F to give it a face.
Go into front view and move the floor down. Press the G KEY to grab it.