Like the ballet heel, another challenging shoe is the heel-less heel. Imagine a platform heel, take away the back heel and you're left with a heel-less heel. The heel-less heel is unique from any other heel in that your body weight must be shifted forward onto the ball of the foot.
As you walk in a heel-less heel, there cannot be a heel strike which means steps must be narrow and each step must be very precise. Since your body weight is shifted to the front of the foot, this decreases your ankle stability which means your posture and your ab engagement must be greater.