You can dig a ditch to the water table in that case.
If you use crossbowmen and ballistas/catapults like early modern armies used musketmen and cannon, then a star fort would be reasonably effective. You don't have to defend every foot of the wall--the job of the defenders is not to meet the enemy as he climbs up top but to shoot him down before he does. This is where the star fort really shines--in a circular fort there are dead zones where shooters can't see because it's around the curve of the circle. In a star fort shooters posted at any of the vertices have a perfect view of the two walls that extend from them, and if they have adequate rate of fire they can shoot down any attackers that try to take any of those walls.
In short, if you rely on melee fighters to throw the enemy off the walls then star forts are awful. If you rely on archers and crossbowmen instead they are excellent.