Here is a really cool Instructable I threw together one weekend:
Step 1: Building the Base
this is a simple step. Just follow the pictures.
Step 2: Build the Tower
start with a pillar around 5 blocks tall and then, start building the corners. they should be three blocks tall and should look like the picture.
Step 3: Start the Top
in picture #1 you see something sticking out from the central pillar. turn it into a square.
Step 4: Finish the Top
in image three the I put slabs on top of the blocks.
Step 5: Final Touches
i do not have any pictured but, you can put in small floors with you windows.
Eventually I will show you how to build a wall in minecraft that is compatible with this tower. Ill have a new instructable soon hope you enjoyed!