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.
Now we need to create the door to the castle. Press the A KEY to deselect all. All vertices should turn pink.
Click View.
Click Top.
Press the B KEY twice for circle selecting. Select the
vertices at the bottom of this picture. They should turn yellow.
The face should turn pink.
Right Click to exit selecting.
Press the P KEY. Click Selected.
Click the triangles next to Edit
Mode. Click Object Mode.
Right Click the wall you just separated. It should turn pink.
Press the M KEY. Click the second box.
Click OK.
The wall should have just disappeared. It is now on a
separate layer. This will make it easier to work with. Click the
second button in the layer selection area.
Press the A KEY to deselect all vertices.
Press the B KEY twice to select vertices. Select the
vertices at the top. 
Right Click to exit selecting.
Press the X KEY. Click Vertices.
Switch to Front View by clicking
View and then Front View.
Press the A KEY to select all.
Press Shift S. Click Cursor-> Selection.
Press the A KEY to deselect All.
Press Spacebar > Add > Mesh >
Circle. 
Click OK next to 32
vertices. 
Press the S KEY for scaling. Move your mouse away from
the circle to make it larger. 
Press the A KEY to deselect all.
Press the B KEY twice to select. Select the bottom
half of the vertices. 
Right Click to exit selecting mode.
Press the X KEY. Click Vertices.
Press the B KEY twice. Select the 2 vertices at the
bottom of the half circle. 
Right Click to exit selecting mode.
Press the E KEY for extrude. Move your mouse down to
meet the bottom of the rectangle.
Press the B KEY twice to select. Select the 2 bottom
vertices of the triangle. 
Right Click to exit selecting mode.
Press the X KEY. Click Edges.
Press the B KEY twice. Select the bottom vertices on
the right. 
Right Click to exit selecting mode.
Press the F KEY.
Press the A KEY.
Press the B KEY twice. Select the bottom vertices on
the left. 
Right Click to exit selecting mode.
Press the F KEY.
Press the A KEY twice to select all vertices. All vertices should be yellow.
Press Shift F. 
We now want to bring back the rest of the castle.
Press this key on your keyboard. It should be at the top left of your
keyboard. 
You will know if
you pressed the right key if your castle comes back.
Click View.
Click Top.
Press the E KEY for extrude. Click Region.
Now move your mouse toward the
middle of the castle. This should bring your wall back.
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.