You'll need to decide what shape towers you are going to have on your castle. Square towers have the advantage of fitting neatly against wall sections, whereas round towers do not fit so well. On the other hand a round tower can be easily be made from a cardboard tube, but a square tower, will need to be made in much the same way as wall sections.

The best way to make a round tower is to use a cardboard tube that is higher than your wall section. Big thick poster tubes, chopped into sections are ideal for this. To make a battlement on top measure the distance around the edge and make the battlement as normal but before you stick it on bend it into shape first. Do not worry about the creases in the cardboard because they will look all right when the tower is textured. If you are not very good at making battlements or can't be bothered just make a tall conical roof to go on top. You may want to add arrow slits or doors to the outside to make it look even more realistic.

Square towers can be given battlements by cutting a crenellation into the top edge. The towers should be taller and thicker than the wall sections although they are made in the same way. Good dimensions for your tower would be about 4-6" square and 6-10" tall depending on the size of your castle. To build a square tower just build a box of the right dimensions, add battlements at the top and any arrowslits or doors and then texture and paint it. Remember, because of its shape, the walls will join it at right angles, whereas with a round tower the walls will join together at various different angles.

The top of the tower must be flat to provide a platform for your models to stand on when defending the castle. This platform could be covered in squares of card to represent flagstones or strips of balsa wood to represent planks or, for simplicity's sake, just paint it gray. A square piece of card could be stuck on and painted to represent a trapdoor. Attach any battlements in the same way as on the wall sections no the 4 sides of the tower.