This is a castle i made using 3D max, i created the towers by creating a standard primitive box then converting it to an editable mesh then editing to shape then i used the taper tool to give the tower a curve, I then used the extrude tool to create raised turrets on my tower.
i did the same with the walls of my castle but i did not taper the walls. If i had more time i would have added more detail to the walls and tower i would of added a door and a draw bridge and also a motte.
This is an exaple of a proffetional 3D design of a castle . as you can see there are allot more sections to this castle in comparison to the one i created. the castle also contains an environment is the form of a motte and land scape with a plain blackdrop. the walls and roof of the castle contain texture and make the modle look more realistic.