Build an Arch
Sorry! Figure it out for yourself (Hint: start with two towers), take a lesson, buy my new book - or download the SoB eBook. (I can't give everything away -- this is how I make my living!)
Carving the Structures You Have Built
Put a point on a tower -- Start at the top by cutting an upside down V so it resembles a steeply sloped roof. Repeat from other angle to form a pyramid. Carefully shave off corners to form a cone shape. Undercut to create roof line. Carve windows and doors, etc. etc.