A battle usually opened with light troops skirmishing with the opposition. These light forces then withdrew to the flanks or between the gaps in the central line of heavy infantry. Cavalry might be launched against their opposing numbers or used to screen the central core from envelopment. As the gap between the contenders closed, the heavy infantry typically took the initiative, attacking on the double. The front ranks usually cast their pila, and the following ranks hurled theirs over the heads of the front-line fighters. If a cast pilum did not cause direct death or injury, they were so designed that the hard iron triangular points would stick into enemy shields, bending on their soft metal shafts, weighing down the shields and making them unusable.