art 1 (the wall)
This is the base of the tower.
Part 2 (parapet)
A variation of the standard box procedure. The white area of the floor only indicates where the wall has to be glued on. You can cut out the windows, if you want (I don't do it as I'm content with the "black windows"). Glue the parapet on top of the wall.
Part 3 (parapet inlay)
Part 3 is only needed, if you cut out the windows of Part 2.
It is a standard box shape, the windows have to be cut out. The inlay walls are not glued to each other but to the inside of the turret.
Part 4 (parapet roof)
Standard roof procedure
You can add as many buttresses as you like. For a corner tower two on adjacent sides are a good idea, as this leaves the other two sides free for connecting the city wall sections. Standard buttress procedure
And that's that.