Ok,you got 2 options to make a circle.
NO1:
start with your base
then select cube tool and select torus in the right colum
push enter and your torus properties comes up
be sure to set your rotation arc at 360,the more sides you put on the circle the better it will look.if you want a rounded top on your circular wall then click circle button,then click OK
you have a circle,then click the arrow tool and drag it to the height you want the wall..
NO2:
make your base then use cube tool and select arch
draw your area you want your circle in and set the height you want your wall
hit enter and your properties page comes up again.Set your sides to 64 and be sure to select 360 in your arc
then texture it how you want,another circle.