How does the cerebral arterial circle ensure that the brain has a constant supply of blood?

Via collateral circulation. This is an alternative route of blood circulation around the brain whenever one pathway is blocked or narrowed. The arterial arrangement of the brain are symmetrically designed in such a way that continuous blood flow the brain is ensured. The cerebral arteries (anterior and posterior) can be interchangeably shared if one of them is restricted. This means that if one artery is blocked/narrowed the other can accommodate the blood flow needed by the brain.